Polylactide compositions and uses thereof

ABSTRACT

The present invention provides compositions and methods relating to polylactides which may be used for drug delivery (e.g., parenteral delivery), wherein an organic solvent is not required.

This application is a divisional of co-pending U.S. patent applicationSer. No. 14/258,357, filed Apr. 22, 2014, which is a divisional of U.S.patent application Ser. No. 13/778,038, filed Feb. 26, 2013, now U.S.Pat. No. 8,741,877, which is a continuation of U.S. patent applicationSer. No. 11/912,102, filed Apr. 23, 2008, now U.S. Pat. No. 8,466,133,which is a national phase application under 35 U.S.C. §371 ofInternational Application No. PCT/IB2006/002849, filed Apr. 21, 2006,which claims the benefit of U.S. Provisional Application Ser. No.60/674,103 filed Apr. 22, 2005 and U.S. Provisional Application Ser. No.60/750,141 filed Dec. 14, 2005, the entire contents and disclosures ofwhich are specifically incorporated by reference herein withoutdisclaimer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the fields of pharmaceuticsand drug delivery. More particularly, it concerns alkyl substitutedpolylactides which may be used to deliver a drug without the need for anorganic solvent.

2. Description of Related Art

Biocompatible and biodegradable polylactides/glycolides (PLA/PLGA) havereceived high attention over the last thirty years in the biomedicalfield as sutures, implants, colloidal drug delivery systems (Penning etal., 1993; Uhrich et al., 1999), and more recently also in tissuerepairing and engineering (Liu and Ma, 2004; Stock and Mayer, 2001) andanti-cancer drug delivery (Mu and Feng, 2003; Jiang et al., 2005). Nextto the medical field they are also widely used in the packaging area. Asbiodegradable “green polymers” they are preferable to the commoditypolymers currently used (Drumright et al., 2000; Vink et al., 2003).

There is a crucial need of well-defined polylactide-based materials withadvanced properties to fit all the requirements for the differentapplications. For example, PLA/PLGA homo- and co-polymers synthesized bythe well-established ring opening polymerization (ROP) process(Dechy-Cabaret et al., 2004; Kricheldorf et al., 1995; Schwach et al.,1997; Degee et al., 1999; Ryner et al., 2001) have a glass transitiontemperature (T_(g)) limited to a range of only 40-60° C. (Jamshidi etal., 1988; Vert et al., 1984), independent of the polymer molecularweight and chemical composition. This combined with interestingmechanical properties makes them suitable in medical applications asbiodegradable implants, bone fracture fixation devices, scaffolds forliving cells.

These polylactides, however, have significant limitations for drugdelivery purposes. For drug delivery purposes, polylactides need to beformulated with organic solvents and administered as solutions or inform of nano- and micro-particles, and polylactides can not be injectedon their own. Thus there is a significant need for a polylactide whichmay be used for drug delivery that does not require the use of anorganic solvent or to form nano- and micro-particles.

SUMMARY OF THE INVENTION

The present invention provides compositions and methods relating topolylactides which may be used for drug delivery which do not requirethe use of an organic solvent or to form nano- and micro-particles priorto injection. These polylactides may be used, for example, to administera drug to a subject (e.g., a human patient) parenterally without the useof a solvent.

An aspect of the present invention relates to a method of preparing apharmaceutical preparation comprising admixing a drug with an alkylsubstituted polylactide; wherein the alkyl substituted polylactide isviscous; and wherein a solvent is not required for said admixing. Thepharmaceutical preparation may be injectable. The pharmaceuticalpreparation may be formulated for parenteral administration to asubject. The subject may be a mammal, such as a human, a mouse, rat, asheep, a goat, a horse, a dog, a cat, a monkey, a cow, or a pig.

In certain embodiments, the alkyl substituted polylactide has thestructure:

wherein R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting of unsubstituted alkyl, H, alkenyl and unsubstitutedalkylaryl; wherein n is 1 to 100; wherein X is hydrogen, —C(O)—CH═CH₂ orany other functional or crosslinking group; and Y is selected from thegroup consisting of —OH, an alkoxy, benzyloxy and—O—(CH₂—CH₂—O)_(p)—CH₃, and wherein p is 1 to 700. In certainembodiments, n is 1 to 100, more preferably 1 to 75, more preferably 1to 50, more preferably 1 to 25, more preferably 1 to 10. In certainembodiments, R¹ and R³ are hydrogen and R² and R⁴ are lower alkyl. Forexample, R² and R⁴ may be —(CH₂)_(m)—CH₃, wherein m is from 0 to 20. Incertain embodiments, m is from 0 to 12.

The alkyl substituted polylactide may be synthesized from alkylsubstituted lactide monomers, for example,3-Methyl-6-hexyl-1,4-dioxane-2,5-dione,3-6-Dihexyl-1,4-dioxane-2,5-dione,3,6,6-Trimethyl-1,4-dioxane-2,5-dione,3-Methyl-6-isopropyl-1,4-dioxane-2,5-dione,3-Methyl-6-butyl-1,4-dioxane-2,5-dione,3-Benzyl-6-methyl-1,4-dioxane-2,5-dione. In certain embodiments, thealkyl substituted polylactide has 1-, 2-, 3- or 4 substituents on thelactide-repeating-unit in the polymer chain. In certain embodiments, thealkyl substituted polylactide has 1-, 2-, 3- or 4 substituents on thelactide-repeating-unit in the polymer chain (i.e., 1, 2, 3 or all of R¹,R², R³ and R⁴ are substituents that are not —H).

The solvent may not be used for said admixing, or the solvent may beused for said admixing. The solvent may be an organic solvent.

In certain embodiments, the polylactide is made by the process ofsubjecting a compound to chemical reaction, wherein the compound has thestructure:

wherein R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting of unsubstituted alkyl, H, alkenyl and unsubstitutedalkylaryl. The chemical reaction may be a ring opening polymerization(ROP). An organic catalyst or an inorganic catalyst may be used in saidROP. The organic catalyst may be tin(II) 2-ethylhexanoate (Sn(Oct)₂),tin(II)trifluoromethane sulfonate (Sn(OTf)₂), 4-(dimethylamino)pyridine(DMAP). In certain embodiments, an alcohol initiator is used in the ROP.The alcohol initiator may be benzyl alcohol, methoxy-poly(ethyleneglycol) (MPEG), 1,1,1-tris(hydroxymethyl)ethane (TE) or pentaerythritol(PE) or any other multi-hydroxy compound. In certain embodiments analcohol initiator is not used in said ROP. In certain embodiments, thepolylactide is acrylated or functionalized with a crosslinkable group.

In certain embodiments, the compound is3-Methyl-6-hexyl-1,4-dioxane-2,5-dione,3-6-Dihexyl-1,4-dioxane-2,5-dione,3,6,6-Trimethyl-1,4-dioxane-2,5-dione,3-Methyl-6-isopropyl-1,4-dioxane-2,5-dione,3-Methyl-6-butyl-1,4-dioxane-2,5-dione or3-Benzyl-6-methyl-1,4-dioxane-2,5-dione.

Another aspect of the present invention relates to a compositionsuitable for parenteral administration, wherein the compositioncomprises an alkyl substituted polylactide, and wherein the alkylsubstituted polylactide is viscous and has the structure:

-   -   wherein R¹, R², R³, and R⁴ are each independently chosen from        the group consisting of unsubstituted alkyl, H, alkenyl and        unsubstituted alkylaryl; wherein n is 1 to 100; wherein X is        hydrogen or —C(O)—CH═CH₂ or any other functional or crosslinking        group; and Y is selected from the croup consisting of —OH, an        alkoxy, benzyloxy or —O—(CH₂—CH₂—O)_(p)—CH₃; and wherein p is 1        to 700, more preferably 1 to 250. In certain embodiments, n is 1        to 100, more preferably 1 to 75, more preferably 1 to 50. In        certain embodiments, R¹ and R³ are hydrogen and R² and R⁴ are        lower alkyl. For example, R² and R⁴ may be —(CH₂)_(m)—CH₃,        wherein m is from 0 to 20. In certain embodiments, m is from 0        to 12.

In certain embodiments, the alkyl substituted polylactide may besynthesized from alkyl substituted lactide monomers including:3-Methyl-6-hexyl-1,4-dioxane-2,5-dione,3-6-Dihexyl-1,4-dioxane-2,5-dione,3,6,6-Trimethyl-1,4-dioxane-2,5-dione,3-Methyl-6-isopropyl-1,4-dioxane-2,5-dione,3-Methyl-6-butyl-1,4-dioxane-2,5-dione,3-Benzyl-6-methyl-1,4-dioxane-2,5-dione. In certain embodiments, thealkyl substituted polylactide has 1, 2, 3 or 4 substituents on thelactide-repeating-unit in the polymer chain (i.e., 1, 2, 3 or all of R¹,R², R³ and R⁴ are substituents that are not —H).

Another aspect of the present invention relates to a compound having thestructure:

wherein Z₂ is selected from the group consisting of —CH₃ and —CH₂—O—Z₅;and wherein Z₁, Z₃, Z₄, and Z₅, each independently has the structure:

wherein R₁, R₂, R₃, and R₄ are each independently chosen from the groupconsisting of unsubstituted alkyl, H, alkenyl and unsubstitutedalkylaryl; wherein n is 1 to 100; wherein X is hydrogen, —C(O)—CH═CH₂ orany other functional or crosslinking group. In certain embodiments, n is1 to 100, more preferably 1 to 75, more preferably 1 to 50. In certainembodiments, R₁ and R₃ are hydrogen; and R₂ and R₄ are lower alkyl. Incertain embodiments, R² and R⁴ are —(CH₂)_(m)—CH₃, wherein m is from 0to 20. In certain embodiments, m is from 0 to 12. In certainembodiments, Z₂ is —CH₃; R₁ and R₃ are hydrogen; R² and R⁴ are—(CH₂)_(m)—CH₃, wherein m is from 0 to 20; and X is hydrogen. In certainembodiments, Z₂ is —CH₃; R₁ and R₃ are hydrogen; R₂ and R₄ are—(CH₂)_(m)—CH₃, wherein m is from 0 to 12; and X is —C(O)—CH═CH₂ or anyother functional or crosslinking group. In certain embodiments, Z₂ is—CH₂—O—Z₅; R₁ and R₃ are hydrogen; R² and R⁴ are —(CH₂)_(m)—CH₃, whereinm=0 or m=5; and X is hydrogen. In certain embodiments, Z₂ is —CH₂—O—Z₅;R₁ and R₃ are hydrogen; R² and R⁴ are —(CH₂)_(m)—CH₃, wherein m=0 orm=5; and X is —C(O)—CH═CH₂ or any other functional or crosslinkinggroup.

Another aspect of the present invention relates to a compound having thestructure:

wherein R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting of unsubstituted alkyl, H, alkenyl and unsubstitutedalkylaryl; wherein n is 1 to 100; wherein X is hydrogen, —C(O)—CH═CH₂ orany other functional or crosslinking group; and Y is—O—(CH₂—CH₂—O)_(p)—CH₃; wherein p is 1 to 700. In certain embodiments, nis 1 to 100, more preferably 1 to 75, more preferably 1 to 50. p may be1 to 700, more preferably 1 to 250. In certain embodiments, R₁ and R₃are hydrogen; and R₂ and R₄ are lower alkyl. In certain embodiments, R²and R⁴ are —(CH₂)_(m)—CH₃, wherein m is from 0 to 20. In certainembodiments, m is from 0 to 12.

Another aspect of the present invention relates to a polylactide made bythe process of subjecting a compound to a ring opening polymerization(ROP) in the presence of an alcohol initiator, wherein the compound hasthe structure:

wherein R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting of unsubstituted alkyl, H, alkenyl and unsubstitutedalkylaryl; and wherein the alcohol initiator is benzyl alcohol,methoxy-poly(ethylene glycol) (MPEG), 1,1,1-tris(hydroxymethyl)ethane(TE) or pentaerythritol (PE) or any other multi-hydroxy compound. R¹,R², R³, and R⁴ may be lower alkyl. An organic catalyst or inorganiccatalyst may be used in said ROP. The organic catalyst may be tin(II)2-ethylhexanoate (Sn(Oct)₂), tin(II)trifluoromethane sulfonate(Sn(OTf)₂) and/or 4-(dimethylamino)pyridine (DMAP). In certainembodiments, the polylactide is acrylated or functionalized with acrosslinkable group.

Another aspect of the present invention relates to a method of making apolylactide comprising subjecting a compound to a ring openingpolymerization (ROP) in the presence of an alcohol initiator, whereinthe compound has the structure:

wherein R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting of unsubstituted alkyl, H, alkenyl and unsubstitutedalkylaryl; and wherein the alcohol initiator is benzyl alcohol,methoxy-poly(ethylene glycol) (MPEG), 1,1,1-tris(hydroxymethyl)ethane(TE) or pentaerythritol (PE) or any other multi-hydroxy compound. R¹,R², R³, and R⁴ may be lower alkyl. An organic catalyst or inorganiccatalyst may be used in said ROP. The organic catalyst may be tin(II)2-ethylhexanoate (Sn(Oct)₂), tin(II)trifluoromethane sulfonate(Sn(OTf)₂) and/or 4-(dimethylamino) pyridine (DMAP) or any othercatalyst. In certain embodiments, the polylactide is acrylated orfunctionalized with any other crosslinkable group.

Another aspect of the present invention relates to a method of treatmentcomprising administering a pharmaceutical composition of the presentinvention to a subject. The composition may be administeredparenterally. The subject may be a mammal, such as a human, a mouse,rat, a sheep, a goat, a horse, a dog, a cat, a monkey, a cow, or a pig.In certain embodiments the composition is injected into the subject.

As used herein the specification, “a” or “an” may mean one or more. Asused herein in the claim(s), when used in conjunction with the word“comprising”, the words “a” or “an” may mean one or more than one. Asused herein “another” may mean at least a second or more.

It is contemplated that any embodiment discussed in this specificationcan be implemented with respect to any method or composition of theinvention, and vice versa. Furthermore, compositions of the inventioncan be used to achieve the methods of the invention.

Throughout this application, the term “about” is used to indicate that avalue includes the inherent variation of error for the device, themethod being employed to determine the value, or the variation thatexists among the study subjects.

The use of the term “or” in the claims is used to mean “and/or” unlessexplicitly indicated to refer to alternatives only or the alternativesare mutually exclusive.

As used in this specification and claim(s), the words “comprising” (andany form of comprising, such as “comprise” and “comprises”), “having”(and any form of having, such as “have” and “has”), “including” (and anyform of including, such as “includes” and “include”), or “containing”(and any form of containing, such as “contains” and “contain”) areinclusive or open-ended and do not exclude additional, unrecitedelements or method steps.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description. It should beunderstood, however, that the detailed description and the specificexamples, while indicating specific embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings form part of the present specification and areincluded to further demonstrate certain aspects of the presentinvention. The invention may be better understood by reference to one ormore of these drawings in combination with the detailed description ofspecific embodiments presented herein.

FIG. 1: Molecular weight versus conversion for the ROP of themonohexyl-substituted lactide mHLA at 100° C. in bulk, with targeted DPof 45 (▪: experimental; □: expected) and 120 (: experimental; ◯:expected) ([BnOH]/[Sn(Oct)₂]=1).

FIGS. 2A-B: Control of the ROP of monohexyl-substituted lactide (FIG.2A) and D,L-lactide (FIG. 2B) performed in bulk or toluene([monomer]/[BnOH]=targeted DP, [BnOH]/[Sn(Oct)₂]=1).

FIG. 3: PmHLA glass transition temperature (T_(g)) as a function of thereciprocal molecular weight (M_(n) ⁻¹).

FIG. 4: PmHLA zero shear viscosity (η₀) at 37° C. (◯) and 25° C. () asa function of the weight average molecular weight (M_(w)).

FIG. 5: Viscosity versus shear rate curves for PmHLA of differentmolecular weights at 25° C.

FIGS. 6A-C: Molecular weight decrease and weight loss of PmHLA () andPLA (◯) of (FIG. 6A) 4500 g/mol, (FIG. 6B) 7500 g/mol, (FIG. 6C) 9100g/mol at 37° C. in phosphate buffer pH 7.4 [For graph (a), benzylester-terminated PmHLA () was compared to the carboxy-terminated PmHLA(▪)].

FIG. 7: Molecular weight decrease and weight loss of PmHLA () and PLA(◯) of 7500 g/mol at 60° C. in phosphate buffer pH 7.4.

FIG. 8: Tetracycline hydrochloride (TH) release from PmHLA () and PLA(◯) (10% w/w drug loading) in phosphate buffer pH 7.4 at 37° C.

FIG. 9: DSC chromatograms of the MPEG-PmHLA copolymers.

FIGS. 10A-B: Plot of the maximum emission wavelength (FIG. 10A) andfluorescence emission intensity (FIG. 10B) of the Nile red vs. copolymerconcentration for MPEG2-PLA3 (◯), MPEG2-PmHLA3 () and MPEG2-PdiHLA3 (▪)micelles.

FIG. 11: Loading of micelles with griseofulvin (mg/g polymer) as afunction of the griseofulvin introduced amount (mg/g polymer) forMPEG2-PLA3 (◯), MPEG2-PmHLA3 () and MPEG2-PdiHLA3 (▪).

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention provides compositions and methods relating topolylactides which may be used for drug delivery which do not requirethe use of an organic solvent or to form nano- and micro-particles priorto injection. These polylactides may be used, for example, to administera drug to a subject (e.g., a human patient) parenterally without the useof a solvent.

Chemical Definitions

An “alkyl” group, as used herein, refers to a saturated aliphatichydrocarbon, including straight-chain, branched chain, and cyclic alkylgroups. Preferably, the alkyl group has 1 to 20 carbons, more preferably1 to 12 carbons, more preferably 1 to 10. Most preferably, it is a loweralkyl of from 1 to 12 carbons. The alkyl groups of the present inventionare preferably unsubstituted. For example, —CH₃, —CH(CH₃)₂ and—(CH₂)_(n)CH₃, wherein n is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,14, 15, 16, 17, 18, 19 or 20 are contemplated alkyl groups that may beused in certain embodiments of the present invention.

An “alkenyl” group, as used herein, refers to an unsaturated aliphatichydrocarbon, including straight-chain, branched chain, and cyclic alkylgroups. Preferably, the alkenyl group has 1 to 20 carbons, morepreferably 1 to 12 carbons, more preferably 1 to 10. Most preferably, itis a lower alkenyl of from 1 to 12 carbons.

An “aryl” group, as used herein, refers to an unsubstituted aromaticgroup which has at least one ring having a conjugated pi electronsystem, and includes carbocyclic aryl, heterocyclic aryl, and biarylgroups. In certain preferred embodiments, the aryl is an unsubstitutedphenyl.

An “alkylaryl” group, as used herein, refers to an alkyl (as describedabove), covalently joined to an aryl group (as described above).Preferably, the alkyl is a lower alkyl. For example, —(CH₂)_(n)(C₆H₅) iscontemplated as an alkylaryl, wherein n is 1 to 20.

An “alkoxy” group refers to an “—O-alkyl” group, where “alkyl” isdefined above.

A “benzyloxy” group, as used herein, refers to the group

“Viscous”, as used herein to describe a polylactide, refers to apolylactide that has a glass transition temperature (Tg) value of lessthan 44 C, more preferably less than 36 C, more preferably less than 35C, more preferably less than 34 C, more preferably less than 33 C, morepreferably less than 32 C, more preferably less than 31 C, morepreferably less than 30 C, more preferably less than 29 C, morepreferably less than 28 C, more preferably less than 27 C, morepreferably less than 26 C, more preferably less than 25 C, morepreferably less than 24 C, more preferably less than 23 C, morepreferably less than 22 C, more preferably less than 21 C, morepreferably less than 20 C, more preferably less than 19 C, morepreferably less than 18 C, more preferably less than 17 C, morepreferably less than 16 C, more preferably less than 15 C, morepreferably less than 14 C, more preferably less than 13 C, morepreferably less than 12 C, more preferably less than 11 C, morepreferably less than 10 C, more preferably less than 9 C, morepreferably less than 8 C, more preferably less than 7 C, more preferablyless than 6 C, more preferably less than 5 C, more preferably less than4 C, more preferably less than 3 C, more preferably less than 2 C, morepreferably less than 1 C, more preferably less than 0 C, more preferablyless than −1 C, more preferably less than −2 C, more preferably lessthan −3 C, more preferably less than −4 C, more preferably less than −5C, more preferably less than −6 C, more preferably less than −7 C, morepreferably less than −8 C, more preferably less than −9 C, mostpreferably less than −10 C.

Polylactides

Polylactides are known in the art. For example, U.S. Pat. No. 6,469,133,U.S. Pat. No. 6,126,919 describe various polylactides and areincorporated by reference herein in their entirety without disclaimer.Polylactides are biodegradable which enhances their utility. Forexample, polylactides may be degraded in the body of a subject (e.g., ahuman patient) into the constituent hydroxycarboxylic acid derivatives(i.e. lactic acids) that form over a period of weeks or years.Polylactides can have molecular weights from about 2000 Da to about250,000 Da. For these reasons, polylactides may be attractive forgenerating things such as degradable sutures, pre-formed implants, andcompounds for drug delivery (e.g., sustained release matrices).

“Alkyl substituted lactide”, as used herein, refers to a compoundcomprising the structure

or a compound having the structure:

wherein R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting of unsubstituted alkyl, H, alkenyl and unsubstitutedalkylaryl.

“Alkyl substituted polylactide”, as used herein, refers to a compoundstructure:

wherein R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting of alkyl (e.g., unsubstituted alkyl), H, alkenyl andalkylaryl (e.g., unsubstituted alkylaryl); wherein X is hydrogen or,alternatively, has been produced as a result of any furtherfunctionalization by chemical reaction on the —OH group formed by the—OX wherein X is hydrogen; Y been derived from any initiator alcohol, orY is selected from the croup consisting of —OH, an alkoxy, benzyloxy and—O—(CH₂—CH₂—O)_(p)—CH₃; and wherein p 1 to 700, more preferably 1 to250; and wherein n is an integer from 1 to 500 or more, more preferably1 to 100, more preferably 1 to 50, more preferably 1 to 25. In certainembodiments, n is from 1 to 12, from 1 to 6, 1, 2, 3, 4, 5, 6, 7, 8, 9,or 10.

In certain embodiments, R¹ and R³ are hydrogen and R² and R⁴ are loweralkyl. For example, R² and R⁴ may be —(CH₂)_(m)—CH₃, wherein m is from 0to 20, more preferably 0 to 15, more preferably 0 to 10, more preferablym=0 or m=5. In certain embodiments, m is from 0 to 6, 0, 1, 2, 3, 4, 5,6, 7, 8, 9, 10, 11 or 12.

In certain embodiments an alkyl substituted polylactide may have thestructure:

wherein Z₂ is selected from the group consisting of —CH₃ and —CH₂—O—Z₅;and wherein Z₁, Z₃, Z₄, and Z₅, each independently has the structure:

wherein R₁, R₂, R₃, and R₄ are each independently chosen from the groupconsisting of alkyl (e.g., unsubstituted alkyl), H, alkenyl andalkylaryl (e.g., unsubstituted alkylaryl); wherein n is 1 to 100;wherein X is hydrogen, —C(O)—CH═CH₂ or any other functional orcrosslinking group. In certain embodiments, n is 1 to 75, morepreferably 1 to 50, more preferably 1 to 25. In certain embodiments, R₁and R₃ are hydrogen; and R₂ and R₄ are lower alkyl. In certainembodiments, R₂ and R₄ are —(CH₂)_(m)—CH₃, wherein m is from 0 to 20. Incertain embodiments, m is from 0 to 20, more preferably 0 to 15, morepreferably 0 to 10, more preferably m=0 or m=5. In certain embodiments,Z₂ is —CH₃; R₁ and R₃ are hydrogen; R₂ and R₄ are —(CH₂)_(m)—CH₃,wherein m is from 0 to 20; and X is hydrogen. In certain embodiments, Z₂is —CH₃; R₁ and R₃ are hydrogen; R₂ and R₄ are —(CH₂)_(m)—CH₃, wherein mis from 0 to 20; and X is —C(O)—CH═CH₂ or any other functional orcrosslinking group. In certain embodiments, Z₂ is —CH₂—O—Z₅; R₁ and R₃are hydrogen; R₂ and R₄ are —(CH₂)_(m)—CH₃, wherein m is from 0 to 20;and X is hydrogen. In certain embodiments, Z₂ is —CH₂—O—Z₅; R₁ and R₃are hydrogen; R² and R⁴ are —(CH₂)_(m)—CH₃, wherein m is from 0 to 20;and X is —C(O)—CH═CH₂. In certain embodiments, m may be from 0 to 20, 0to 16, 0 to 12, or 0 to 6.

In certain embodiments an alkyl substituted polylactide may have thestructure:

wherein R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting of alkyl (e.g., unsubstituted alkyl), H, alkenyl andalkylaryl (e.g., unsubstituted alkylaryl); wherein n is 1 to 100;wherein X is hydrogen or —C(O)—CH═CH₂ or any other functional orcrosslinking group; and Y is —O—(CH₂—CH₂—O)_(p)—CH₃; wherein p is 1 to700, more preferably 1 to 250. In certain embodiments, n is 1 to 100,more preferably 1 to 75, more preferably 1 to 50, more preferably 1 to25, 1 to 12 or 1 to 6. In certain embodiments, R₁ and R₃ are hydrogen;and R₂ and R₃ are lower alkyl. In certain embodiments, R² and R⁴ are—(CH₂)_(m)—CH₃, wherein m is from 0 to 20, more preferably 0 to 6. Incertain embodiments, m is from 0 to 6, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11, 12.

The alkyl substituted polylactides of the present invention may comprisea drug (e.g., a hydrophobic drug or a hydrophilic drug), and the alkylsubstituted polylactides of the present invention may be used to delivera drug to a subject (e.g., a human patient). In certain embodiments, thealkyl substituted polylactides of the present invention are used todeliver a hydrophobic drug, such as tetracycline, to a subject.

Further, the alkyl substituted polylactides of the present invention maybe used to alter the pharmacokinetics of a drug. For example, in certainembodiments, the substituted polylactides of the present invention maybe used to reduce the degradation of a drug. In certain embodiments, thesubstituted polylactides of the present invention may be used to morecompletely (i.e., as compared to PLA) release the drug (e.g., an activeform of the drug) into a subject.

A. Synthesis of Polylactides

Certain polylactides of the present invention may be synthesized by thegeneral synthesis paradigm:

wherein R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting unsubstituted alkyl (e.g., unsubstituted alkyl), H, alkenylor alkylaryl (e.g., unsubstituted alkylaryl). A subsequent ring openingpolymerization (ROP) may be performed using, for example, tin(II)2-ethylhexanoate (Sn(Oct)₂), tin(II)trifluoromethane sulfonate(Sn(OTf)₂), 4-(dimethylamino)pyridine (DMAP), and/or another organiccatalyst. In certain embodiments, Sn(Oct)₂ is used as the catalyst.

For example, ring-opening polymerizations may be performed with the“standard” FDA-approved (Food Drug Admin, 1975) metal organic catalystsSn(Oct)₂, with the more reactive “tin-based” tin(II) trifluoromethanesulfonate Sn(OTf)₂ (Möller et al., 2000; Möller et al., 2001) and/or thesolely organic catalyst 4-(dimethylamino)pyridine DMAP (Nederberg etal., 2001). The “coordination-insertion” mechanism for ROP of cyclicesters has been well established and described, e.g., by Kowalski et al.(1998). Kricheldorf et al. (2000) and others for the most commonly usedSn(Oct)₂ catalyst. In this reaction mechanism Sn(Oct)₂ exchanges atleast one of its 2-ethylhexanoate ligands with the initiating alcohol toform a tin alkoxide initiator. In these embodiments, after monomerring-opening leading to an alcohol ester end group the propagationproceeds through the tin alkoxide active centers. This mechanism canalso apply to the ROP used here in certain embodiments for thepolymerizations of the alkyl-substituted monomers catalyzed by Sn(Oct)₂,Sn(OTf)₂ and DMAP, respectively. The use of benzyl alcohol (BnOH) as thealcohol initiator can enable later further functionalization of thepolymers by deprotection of the benzyl end groups with H₂/Pd. In certainembodiments where steric more hindered monomers cannot be efficientlypolymerized with Sn(Oct)₂ and Sn(OTf)₂, the use of the DMAP catalyst maybe successfully applied. Good control of molecular weight and narrowpolydispersities may be achieved for ROP of, e.g., monoalkyl-substitutedmonomers leading to new functionalized poly(lactides).

In certain embodiments, an alcohol initiator may be used in the ROP. Inother embodiments, an alcohol initiator is not used in the ROP. Alcoholinitiators include: benzyl alcohol, methoxy-poly(ethylene glycol)(MPEG), 1,1,1-tris(hydroxymethyl)ethane (TE) and pentaerythritol (PE).Other alcohol inhibitors that may be used with the present inventioninclude molecules with multiple hydroxy groups.

In certain embodiments, a subsequent acrylation step or afunctionalization with a crosslinking compound may be performed. Forexample, acrylation may be performed by subjecting a polylactide to anexcess of acryloyl chloride. Crosslinking may be achieved by any othercrosslinking agent, wherein the crosslinking groups comprise degradableor nondegradable functionality.

In certain embodiments, a polylactide may be made by the process ofsubjecting a compound to a ring opening polymerization (ROP) in thepresence of an alcohol initiator, wherein the compound has thestructure:

wherein R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting of unsubstituted alkyl, H, alkenyl and unsubstitutedalkylaryl; and wherein the alcohol initiator is benzyl alcohol,methoxy-poly(ethylene glycol) (MPEG), 1,1,1-tris(hydroxymethyl)ethane(TE), pentaerythritol (PE) or a compound with multiple hydroxy groups.R¹, R², R³, and R⁴ may be lower alkyl. An organic catalyst may be usedin said ROP. The organic catalyst may be tin(II) 2-ethylhexanoate(Sn(Oct)₂), tin(II)trifluoromethane sulfonate (Sn(OTf)₂) and/or4-(dimethylamino)pyridine (DMAP). In certain embodiments, thepolylactide is acrylated or functionalized with a crosslinking compound.

In certain embodiments, the compound may have any of the followingstructures:

These compounds may be used to produce an alkyl substituted polylactidewhich has 1, 2, 3 or 4 substituents on the lactide-repeating-unit in thepolymer chain (i.e., 1, 2, 3 or all of R¹, R², R³ and R⁴ (R, R′, R″ andR′″ above) are substituents that are not hydrogen).

B. Use of Polylactides in Combination with Other Compounds

The polylactides of the present invention may be used in combinationwith other polylactides, polyglycolides and their copolymers. Forexample, the polylactides of the present invention may be admixed withor contacted with a second compound and the resulting composition may beused for drug delivery. Compounds which may be used as the secondcompound or in combination with the polylactides of the presentinvention include polyglycolide (PLGA), polylactic acid (PLA),polycaprolactone (PCL), polyethylene glycol (PEG), polydioxanone (PDO),poly(D,L-lactide-co-glycolide) and poly(L-lactide-co-glycolide),poly(hydroxyl alkanoate) (PHA), and biodegradable and biocompatiblepolymers. Biocompatible polymers include polyester, polyether,polyanhydride, polyamines, poly(ethylene imines) polyamides,polyesteramides, polyorthoesters, polydioxanones, polyacetals,polyketals, polycarbonates, polyphosphoesters, polybutylene,polyterephthalate, polyorthocarbonates, polyphosphazenes, polyurethanes,polytetrafluorethylenes (PTFE), polysuccinates, poly(malic acid),poly(amino acids), polyvinylpyrrolidone, polyhydroxycellulose,polysaccharides, chitin, chitosan, hyaluronic acid, and copolymers,terpolymers and mixtures thereof. In certain embodiments, syntheticpolymers and/or natural polymers (e.g., as listed below) may be used asthe second compound or in combination with polylactides of the presentinvention.

Synthetic Polymers

Degradable: Non-Degradable: Poly(glycolic) acid and Poly(lactic acid)Poly acrylic acids in general: Poly(hydroxyl alkanoates) Polymethylmethacrylate (PMMA) (PHAs) Poly acryl amides Poly capro-, butyro-,valero-lactones Poly acrylo nitriles/ = cyano acrylates Poly orthoestersPoly functionalized methacrylic acids Poly anhydrides Poly urethanesPoly carbonates Poly olefins Polyester from alcanoic acids + Polystyrene dialcohols Poly terephthalates Poly amides Poly ethylenes,propylenes Poly imides Poly ether ketones Poly imines Polyvinylchlorides Poly imino carbonates Poly fluorides Poly ethylene iminesPoly PTFE Poly dioxanes Silicones Poly phosphazenes Poly silicates(bioactive glass) Poly sulphones Siloxanes (Poly dimethyl SyntheticLipids siloxanes)

Natural Polymers:

Poly(aminoacids) (natural and (non natural poly β-aminoesters)) e.g.:Poly (aspartic acid), - (glutamic acid), -(lysine), -(histidine)Poly(peptides) and proteins Poly and oligo nucleic acids AlbuminesAlginates Cellulose/Cellulose acetates Chitin/Chitosan CollageneFibrine/Fibrinogen Gelatine Lignine Poly(hyaluronic acids)Poly(hydroxyalkanoates) Polyisoprenoids Polysaccharides Starch basedpolymers

C. Use of Polylactides in Combination with Plasticizers

In certain embodiments it may be desirable to contact or admix an alkylsubstituted polylactide with one or more plasticizers, in order to alterthe physical properties (e.g., lowering the T_(g)) of the resultingcomposition. Plasticizers which may be used in combination with an alkylsubstituted polylactide include all FDA approved plasticizers, such asbenzyl benzoates, cellulose acetates, cellulose acetate phthalates,chlorobutanol, dextrines, dibutyl sebacate, dimethyl sebacate, acetylphthalates, diethyl phthalate dibutyl phthalate, dipropyl phthalate,dimethyl phthalate, dioctyl phthalate, methyl cellulose, ethylcellulose, hydroxylethyl cellulose, hydroxypropyl cellulose,hydroxypropyl methyl celluloses, gelatine, glycerines, glycerylmonostearate, monoglycerides, mono and di-acetylated monoglycerides,glycerol, mannitol, mineral oils and lanolin alcohols, petrolatum andlanolin alcohols, castor oil, vegetable oils, coconut oil, polyethyleneglycol, polymethacrylates and copolymers thereof, polyvinylpyrrolidone,propylene carbonates, propylene glycol, sorbitol, suppository bases,diacetine, triacetin, triethanolamine, esters of citric acid, triethylcitrate, acetyl triethyl citrate, acetyl tributyl citrate, triethylcitrate, esters of phosphoric acid.

For example, certain alkyl substituted polylactides of the presentinvention (e.g., polylactides with higher molecular weights) may be waxyand thus not injectable. However, these alkyl substituted polylactidesmay still retain the very desirable property of being very hydrophobicin comparison to normal PLA/PLGA, thus having an advantage for manypharmaceutical applications. An increased hydrophobic drug incorporationinto the alkyl substituted polylactide due to the increasedhydrophobicity of the polylactide. Certain alkyl substitutedpolylactides of the present invention (e.g., polylactides with highermolecular weights) may exhibit better control of drug release. Thus, incertain embodiments a non-injectable alkyl substituted polylactide couldbe made injectable by admixing a plasticizer with the polylactide.

Pharmaceutical Preparations

Pharmaceutical compositions of the present invention comprise aneffective amount of one or more alkyl substituted polylactide oradditional agent dissolved in, dispersed in, or used as apharmaceutically acceptable carrier. Further it is recognized that oneor more alkyl substituted polylactide may be used in combination with anadditional agent in or as a pharmaceutically acceptable carrier.

The phrases “pharmaceutical or pharmacologically acceptable” refers tomolecular entities and compositions that do not produce an adverse,allergic or other untoward reaction when administered to an animal, suchas, for example, a human, as appropriate. The preparation of anpharmaceutical composition that contains at least one alkyl substitutedpolylactide or additional active ingredient will be known to those ofskill in the art in light of the present disclosure, as exemplified byRemington's Pharmaceutical Sciences, 18th Ed. Mack Printing Company,1990, incorporated herein by reference. Moreover, for animal (e.g.,human) administration, it will be understood that preparations shouldmeet sterility, pyrogenicity, general safety and purity standards asrequired by FDA Office of Biological Standards.

As used herein, “pharmaceutically acceptable carrier” includes any andall solvents, dispersion media, coatings, surfactants, antioxidants,preservatives (e.g., antibacterial agents, antifungal agents), isotonicagents, absorption delaying agents, salts, preservatives, drugs, drugstabilizers, gels, binders, excipients, disintegration agents,lubricants, sweetening agents, flavoring agents, dyes, such likematerials and combinations thereof, as would be known to one of ordinaryskill in the art (see, for example, Remington's Pharmaceutical Sciences,18th Ed. Mack Printing Company, 1990, pp. 1289-1329, incorporated hereinby reference). Except insofar as any conventional carrier isincompatible with the active ingredient, its use in the pharmaceuticalcompositions is contemplated.

The alkyl substituted polylactide may comprise different types ofcarriers depending on whether it is to be administered in solid, liquidor aerosol form, and whether it need to be sterile for such routes ofadministration as injection. The present invention can be administeredintravenously, intradermally, transdermally, intrathecally,intraarterially, intraperitoneally, intranasally, intravaginally,intrarectally, topically, intramuscularly, subcutaneously, mucosally,orally, topically, locally, inhalation (e.g., aerosol inhalation),injection, infusion, continuous infusion, localized perfusion bathingtarget cells directly, via a catheter, via a lavage, in cremes, in lipidcompositions (e.g., liposomes), or by other method or any combination ofthe forgoing as would be known to one of ordinary skill in the art (see,for example, Remington's Pharmaceutical Sciences, 18th Ed. Mack PrintingCompany, 1990, incorporated herein by reference).

The alkyl substituted polylactide may be formulated into a compositionin a free base, neutral or salt form. Pharmaceutically acceptable salts,include the acid addition salts, e.g., those formed with the free aminogroups of a proteinaceous composition, or which are formed withinorganic acids such as for example, hydrochloric or phosphoric acids,or such organic acids as acetic, oxalic, tartaric or mandelic acid.Salts formed with the free carboxyl groups can also be derived frominorganic bases such as for example, sodium, potassium, ammonium,calcium or ferric hydroxides; or such organic bases as isopropylamine,trimethylamine, histidine or procaine. Upon formulation, solutions willbe administered in a manner compatible with the dosage formulation andin such amount as is therapeutically effective. The formulations areeasily administered in a variety of dosage forms such as formulated forparenteral administrations such as injectable solutions, or aerosols fordelivery to the lungs, or formulated for alimentary administrations suchas drug release capsules and the like.

Further in accordance with the present invention, the composition of thepresent invention suitable for administration is provided in apharmaceutically acceptable carrier with or without an inert diluent.The carrier should be assimilable and includes liquid, semi-solid, i.e.,pastes, or solid carriers. Except insofar as any conventional media,agent, diluent or carrier is detrimental to the recipient or to thetherapeutic effectiveness of a the composition contained therein, itsuse in administrable composition for use in practicing the methods ofthe present invention is appropriate. Examples of carriers or diluentsinclude fats, oils, water, saline solutions, lipids, liposomes, resins,binders, fillers and the like, or combinations thereof. The compositionmay also comprise various antioxidants to retard oxidation of one ormore component. Additionally, the prevention of the action ofmicroorganisms can be brought about by preservatives such as variousantibacterial and antifungal agents, including but not limited toparabens (e.g., methylparabens, propylparabens), chlorobutanol, phenol,sorbic acid, thimerosal or combinations thereof.

In accordance with the present invention, the composition is combinedwith the carrier in any convenient and practical manner, i.e., bysolution, suspension, emulsification, admixture, encapsulation,absorption and the like. Such procedures are routine for those skilledin the art.

In a specific embodiment of the present invention, the composition iscombined or mixed thoroughly with a semi-solid or solid carrier. Themixing can be carried out in any convenient manner such as grinding.Stabilizing agents can be also added in the mixing process in order toprotect the composition from loss of therapeutic activity, i.e.,denaturation in the stomach. Examples of stabilizers for use in an thecomposition include buffers, amino acids such as glycine and lysine,carbohydrates such as dextrose, mannose, galactose, fructose, lactose,sucrose, maltose, sorbitol, mannitol, etc.

In further embodiments, the present invention may concern the use of apharmaceutical lipid vehicle compositions that include alkyl substitutedpolylactide, one or more lipids, and an aqueous solvent. As used herein,the term “lipid” will be defined to include any of a broad range ofsubstances that is characteristically insoluble in water and extractablewith an organic solvent. This broad class of compounds are well known tothose of skill in the art, and as the term “lipid” is used herein, it isnot limited to any particular structure. Examples include compoundswhich contain long-chain aliphatic hydrocarbons and their derivatives. Alipid may be naturally occurring or synthetic (i.e., designed orproduced by man). However, a lipid is usually a biological substance.Biological lipids are well known in the art, and include for example,neutral fats, phospholipids, phosphoglycerides, steroids, terpenes,lysolipids, glycosphingolipids, glycolipids, sulphatides, lipids withether and ester-linked fatty acids and polymerizable lipids, andcombinations thereof. Of course, compounds other than those specificallydescribed herein that are understood by one of skill in the art aslipids are also encompassed by the compositions and methods of thepresent invention.

One of ordinary skill in the art would be familiar with the range oftechniques that can be employed for dispersing a composition in a lipidvehicle. For example, the alkyl substituted polylactide may be dispersedin a solution containing a lipid, dissolved with a lipid, emulsifiedwith a lipid, mixed with a lipid, combined with a lipid, covalentlybonded to a lipid, contained as a suspension in a lipid, contained orcomplexed with a micelle or liposome, or otherwise associated with alipid or lipid structure by any means known to those of ordinary skillin the art. The dispersion may or may not result in the formation ofliposomes.

The actual dosage amount of a composition of the present inventionadministered to an animal patient can be determined by physical andphysiological factors such as body weight, severity of condition, thetype of disease being treated, previous or concurrent therapeuticinterventions, idiopathy of the patient and on the route ofadministration. Depending upon the dosage and the route ofadministration, the number of administrations of a preferred dosageand/or an effective amount may vary according to the response of thesubject. The practitioner responsible for administration will, in anyevent, determine the concentration of active ingredient(s) in acomposition and appropriate dose(s) for the individual subject.

In certain embodiments, pharmaceutical compositions may comprise, forexample, at least about 0.1% of an active compound. In otherembodiments, the an active compound may comprise between about 2% toabout 75% of the weight of the unit, or between about 25% to about 60%,for example, and any range derivable therein. Naturally, the amount ofactive compound(s) in each therapeutically useful composition may beprepared is such a way that a suitable dosage will be obtained in anygiven unit dose of the compound. Factors such as solubility,bioavailability, biological half-life, route of administration, productshelf life, as well as other pharmacological considerations will becontemplated by one skilled in the art of preparing such pharmaceuticalformulations, and as such, a variety of dosages and treatment regimensmay be desirable.

In other non-limiting examples, a dose may also comprise from about 1microgram/kg/body weight, about 5 microgram/kg/body weight, about 10microgram/kg/body weight, about 50 microgram/kg/body weight, about 100microgram/kg/body weight, about 200 microgram/kg/body weight, about 350microgram/kg/body weight, about 500 microgram/kg/body weight, about 1milligram/kg/body weight, about 5 milligram/kg/body weight, about 10milligram/kg/body weight, about 50 milligram/kg/body weight, about 100milligram/kg/body weight, about 200 milligram/kg/body weight, about 350milligram/kg/body weight, about 500 milligram/kg/body weight, to about1000 mg/kg/body weight or more per administration, and any rangederivable therein. In non-limiting examples of a derivable range fromthe numbers listed herein, a range of about 5 mg/kg/body weight to about100 mg/kg/body weight, about 5 microgram/kg/body weight to about 500milligram/kg/body weight, etc., can be administered, based on thenumbers described above.

A. Alimentary Compositions and Formulations

In preferred embodiments of the present invention, the alkyl substitutedpolylactide are formulated to be administered via an alimentary route.Alimentary routes include all possible routes of administration in whichthe composition is in direct contact with the alimentary tract.Specifically, the pharmaceutical compositions disclosed herein may beadministered orally, buccally, rectally, or sublingually. As such, thesecompositions may be formulated with an inert diluent or with anassimilable edible carrier, or they may be enclosed in hard- orsoft-shell gelatin capsule, or they may be compressed into tablets, orthey may be incorporated directly with the food of the diet.

In certain embodiments, the active compounds may be incorporated withexcipients and used in the form of ingestible tablets, buccal tables,troches, capsules, elixirs, suspensions, syrups, wafers, and the like(Mathiowitz et al., 1997; Hwang et al., 1998; U.S. Pat. Nos. 5,641,515;5,580,579 and 5,792,451, each specifically incorporated herein byreference in its entirety). The tablets, troches, pills, capsules andthe like may also contain the following: a binder, such as, for example,gum tragacanth, acacia, cornstarch, gelatin or combinations thereof; anexcipient, such as, for example, dicalcium phosphate, mannitol, lactose,starch, magnesium stearate, sodium saccharine, cellulose, magnesiumcarbonate or combinations thereof; a disintegrating agent, such as, forexample, corn starch, potato starch, alginic acid or combinationsthereof; a lubricant, such as, for example, magnesium stearate; asweetening agent, such as, for example, sucrose, lactose, saccharin orcombinations thereof; a flavoring agent, such as, for examplepeppermint, oil of wintergreen, cherry flavoring, orange flavoring, etc.When the dosage unit form is a capsule, it may contain, in addition tomaterials of the above type, a liquid carrier. Various other materialsmay be present as coatings or to otherwise modify the physical form ofthe dosage unit. For instance, tablets, pills, or capsules may be coatedwith shellac, sugar, or both. When the dosage form is a capsule, it maycontain, in addition to materials of the above type, carriers such as aliquid carrier. Gelatin capsules, tablets, or pills may be entericallycoated. Enteric coatings prevent denaturation of the composition in thestomach or upper bowel where the pH is acidic. See, e.g., U.S. Pat. No.5,629,001. Upon reaching the small intestines, the basic pH thereindissolves the coating and permits the composition to be released andabsorbed by specialized cells, e.g., epithelial enterocytes and Peyer'spatch M cells. A syrup of elixir may contain the active compound sucroseas a sweetening agent methyl and propylparabens as preservatives, a dyeand flavoring, such as cherry or orange flavor. Of course, any materialused in preparing any dosage unit form should be pharmaceutically pureand substantially non-toxic in the amounts employed. In addition, theactive compounds may be incorporated into sustained-release preparationand formulations.

For oral administration the compositions of the present invention mayalternatively be incorporated with one or more excipients in the form ofa mouthwash, dentifrice, buccal tablet, oral spray, or sublingualorally-administered formulation. For example, a mouthwash may beprepared incorporating the active ingredient in the required amount inan appropriate solvent, such as a sodium borate solution (Dobell'sSolution). Alternatively, the active ingredient may be incorporated intoan oral solution such as one containing sodium borate, glycerin andpotassium bicarbonate, or dispersed in a dentifrice, or added in atherapeutically-effective amount to a composition that may includewater, binders, abrasives, flavoring agents, foaming agents, andhumectants. Alternatively the compositions may be fashioned into atablet or solution form that may be placed under the tongue or otherwisedissolved in the mouth.

Additional formulations which are suitable for other modes of alimentaryadministration include suppositories. Suppositories are solid dosageforms of various weights and shapes, usually medicated, for insertioninto the rectum. After insertion, suppositories soften, melt or dissolvein the cavity fluids. In general, for suppositories, traditionalcarriers may include, for example, polyalkylene glycols, triglyceridesor combinations thereof. In certain embodiments, suppositories may beformed from mixtures containing, for example, the active ingredient inthe range of about 0.5% to about 10%, and preferably about 1% to about2%.

B. Parenteral Compositions and Formulations

In further embodiments, an alkyl substituted polylactide may beadministered via a parenteral route. As used herein, the term“parenteral” includes routes that bypass the alimentary tract.Specifically, the pharmaceutical compositions disclosed herein may beadministered for example, but not limited to intravenously,intradermally, intramuscularly, intraarterially, intrathecally,subcutaneous, or intraperitoneally U.S. Pat. Nos. 6,753,514, 6,613,308,5,466,468, 5,543,158; 5,641,515; and 5,399,363 (each specificallyincorporated herein by reference in its entirety).

Solutions of the active compounds as free base or pharmacologicallyacceptable salts may be prepared in water suitably mixed with asurfactant, such as hydroxypropylcellulose. Dispersions may also beprepared in glycerol, liquid polyethylene glycols, and mixtures thereofand in oils. Under ordinary conditions of storage and use, thesepreparations contain a preservative to prevent the growth ofmicroorganisms. The pharmaceutical forms suitable for injectable useinclude sterile aqueous solutions or dispersions and sterile powders forthe extemporaneous preparation of sterile injectable solutions ordispersions (U.S. Pat. No. 5,466,468, specifically incorporated hereinby reference in its entirety). In all cases the form must be sterile andmust be fluid to the extent that easy injectability exists. It must bestable under the conditions of manufacture and storage and must bepreserved against the contaminating action of microorganisms, such asbacteria and fungi. The carrier can be a solvent or dispersion mediumcontaining, for example, water, ethanol, polyol (i.e., glycerol,propylene glycol, and liquid polyethylene glycol, and the like),suitable mixtures thereof, and/or vegetable oils. Proper fluidity may bemaintained, for example, by the use of a coating, such as lecithin, bythe maintenance of the required particle size in the case of dispersionand by the use of surfactants. The prevention of the action ofmicroorganisms can be brought about by various antibacterial andantifungal agents, for example, parabens, chlorobutanol, phenol, sorbicacid, thimerosal, and the like. In many cases, it will be preferable toinclude isotonic agents, for example, sugars or sodium chloride.Prolonged absorption of the injectable compositions can be brought aboutby the use in the compositions of agents delaying absorption, forexample, aluminum monostearate and gelatin.

For parenteral administration in an aqueous solution, for example, thesolution should be suitably buffered if necessary and the liquid diluentfirst rendered isotonic with sufficient saline or glucose. Theseparticular aqueous solutions are especially suitable for intravenous,intramuscular, subcutaneous, and intraperitoneal administration. In thisconnection, sterile aqueous media that can be employed will be known tothose of skill in the art in light of the present disclosure. Forexample, one dosage may be dissolved in 1 ml of isotonic NaCl solutionand either added to 1000 ml of hypodermoclysis fluid or injected at theproposed site of infusion, (see for example, “Remington's PharmaceuticalSciences” 15th Edition, pages 1035-1038 and 1570-1580). Some variationin dosage will necessarily occur depending on the condition of thesubject being treated. The person responsible for administration will,in any event, determine the appropriate dose for the individual subject.Moreover, for human administration, preparations should meet sterility,pyrogenicity, general safety and purity standards as required by FDAOffice of Biologics standards.

Sterile injectable solutions are prepared by incorporating the activecompounds in the required amount in the appropriate solvent with variousof the other ingredients enumerated above, as required, followed byfiltered sterilization. Generally, dispersions are prepared byincorporating the various sterilized active ingredients into a sterilevehicle which contains the basic dispersion medium and the requiredother ingredients from those enumerated above. In the case of sterilepowders for the preparation of sterile injectable solutions, thepreferred methods of preparation are vacuum-drying and freeze-dryingtechniques which yield a powder of the active ingredient plus anyadditional desired ingredient from a previously sterile-filteredsolution thereof. A powdered composition is combined with a liquidcarrier such as, e.g., water or a saline solution, with or without astabilizing agent.

C. Miscellaneous Pharmaceutical Compositions and Formulations

In other preferred embodiments of the invention, the active compoundalkyl substituted polylactide may be formulated for administration viavarious miscellaneous routes, for example, topical (i.e., transdermal)administration, mucosal administration (intranasal, vaginal, etc.)and/or inhalation.

Pharmaceutical compositions for topical administration may include theactive compound formulated for a medicated application such as anointment, paste, cream or powder. Ointments include all oleaginous,adsorption, emulsion and water-solubly based compositions for topicalapplication, while creams and lotions are those compositions thatinclude an emulsion base only. Topically administered medications maycontain a penetration enhancer to facilitate adsorption of the activeingredients through the skin. Suitable penetration enhancers includeglycerin, alcohols, alkyl methyl sulfoxides, pyrrolidones andluarocapram. Possible bases for compositions for topical applicationinclude polyethylene glycol, lanolin, cold cream and petrolatum as wellas any other suitable absorption, emulsion or water-soluble ointmentbase. Topical preparations may also include emulsifiers, gelling agents,and antimicrobial preservatives as necessary to preserve the activeingredient and provide for a homogenous mixture. Transdermaladministration of the present invention may also comprise the use of a“patch”. For example, the patch may supply one or more active substancesat a predetermined rate and in a continuous manner over a fixed periodof time.

In certain embodiments, the pharmaceutical compositions may be deliveredby eye drops, intranasal sprays, inhalation, and/or other aerosoldelivery vehicles. Methods for delivering compositions directly to thelungs via nasal aerosol sprays has been described e.g., in U.S. Pat.Nos. 5,756,353 and 5,804,212 (each specifically incorporated herein byreference in its entirety). Likewise, the delivery of drugs usingintranasal microparticle resins (Takenaga et al., 1998) andlysophosphatidyl-glycerol compounds (U.S. Pat. No. 5,725,871,specifically incorporated herein by reference in its entirety) are alsowell-known in the pharmaceutical arts. Likewise, transmucosal drugdelivery in the form of a polytetrafluoroethylene support matrix isdescribed in U.S. Pat. No. 5,780,045 (specifically incorporated hereinby reference in its entirety).

The term aerosol refers to a colloidal system of finely divided solid ofliquid particles dispersed in a liquefied or pressurized gas propellant.The typical aerosol of the present invention for inhalation will consistof a suspension of active ingredients in liquid propellant or a mixtureof liquid propellant and a suitable solvent. Suitable propellantsinclude hydrocarbons and hydrocarbon ethers. Suitable containers willvary according to the pressure requirements of the propellant.Administration of the aerosol will vary according to subject's age,weight and the severity and response of the symptoms.

EXAMPLES

The following examples are included to demonstrate preferred embodimentsof the invention. It should be appreciated by those of skill in the artthat the techniques disclosed in the examples which follow representtechniques discovered by the inventor to function well in the practiceof the invention, and thus can be considered to constitute preferredmodes for its practice. However, those of skill in the art should, inlight of the present disclosure, appreciate that many changes can bemade in the specific embodiments which are disclosed and still obtain alike or similar result without departing from the spirit and scope ofthe invention.

Example 1 Synthesis and Ring-Opening Polymerization of NewAlkyl-Substituted Lactides

Biocompatible and biodegradable poly(lactide) materials have receivedhigh interest over the past three decades, initially in the biomedicalfield, e.g as sutures and implants or as drug-delivery systems (Penninget al., 1993; Uhrich et al., 1999). They are also reported and used asenvironmentally friendly packaging materials (Drumright et al., 2000),and commercially applied in areas as wide as foams, apparels, carpets ormattresses (Vink et al., 2003). Based on the lactide monomer, which issynthesized from renewable resources, and thanks to the reduced polymercosts allowed by new industrial scale technologies, biodegradablepoly(lactides) have major advantages to other synthetic polymers.

The ability of modulating the physico-chemical properties of the polymer(hydrophobicity/philicity, degradability, T_(g), etc.) is a key pointfor obtaining materials adapted to their specific application. In thiscontext the design of new controlled polymerizable alkyl-substitutedlactides (referring to lactides, in which at least one of the methylligand is substituted by an other alkyl substituent) can be aninteresting approach to tailor material properties for various medicalapplications by using them as new homopolymers or as new copolymerstogether with lactides and glycolides. Although the ring-openingpolymerization (ROP) of lactides is widely described in the literature(Kricheldorf et al., 1995; Hyon et al., 1997; Schwach et al., 1997;Degee et al., 1999; Ryner et al., 2001; Shirahama et al., 2002; Myers etal., 2002; Ouchi et al., 2002; Storey et al., 2002; Finne et al., 2003;Mullen et al., 2003; Mullen et al., 2003; McGuinness et al., 2003), thesynthesis and the ROP of aliphatic derivatives are hardly investigated(Lou et al., 2003). Only recently Baker et al. reported thepolymerization of the symmetric dibenzyl-substituted lactide3,6-di(phenylmethyl)-1,4-dioxane-2,5-dione (6′ as shown below).Polymerizations carried out in solution at 50-100° C. with tin(II)2-ethylhexanoate (Sn(Oct)₂) as catalyst led to relative low conversions(<70%) even after a reaction time of one week, whereas meltpolymerization at 180° C. allowed 90% conversion within two hours.Nevertheless in this latter case side transesterification andepimerization reactions were observed after prolonged polymerizationtimes (Simmons and Baker, 2001). This group also reported the synthesisof other new symmetric substituted lactides, such as3,6-diethyl-1,4-dioxane-2,5-dione, 3,6-diisobutyl-1,4-dioxane-2,5-dioneand 3,6-dihexyl-1,4-dioxane-2,5-dione (5′) (Yin and Baker, 1999). Thering-opening polymerizations of these monomers in presence of severaltin-based catalysts, with or without alcohol initiator, at relative hightemperatures (130-180° C.) led to polymers showing high polydispersities(>1.7). The substituted lactides syntheses were realized by twodifferent methods, either by the classical condensation of thecorresponding α-hydroxy acid (Deane and Hammond, 1960) or by thetwo-step synthesis of an α-hydroxy acid with a 2-halo-alkanoylhalogenide reported prior by Schollkopf et al. (1979). Baker and Smith(2002) describe an ROP with metal organic catalysts.

Structures of synthesized substituted lactides are shown below; 1:D,L-lactide, 2: 3,6,6-Trimethyl-1,4-dioxane-2,5-dione (referred asdimethyl-substituted lactide), 3:3-Methyl-6-isopropyl-1,4-dioxane-2,5-dione (isopropyl-substitutedlactide), 4: 3-Methyl-6-butyl-1,4-dioxane-2,5-dione (butyl-substitutedlactide), 5: 3-Methyl-6-hexyl-1,4-dioxane-2,5-dione (hexyl-substitutedlactide), 5′: 3,6-Dihexyl-1,4-dioxane-2,5-dione (symmetricdihexyl-substituted lactide), 6:3-Methyl-6-phenylmethyl-1,4-dioxane-2,5-dione (benzyl-substitutedlactide), 6′: 3,6-Diphenylmethyl-1,4-dioxane-2,5-dione (symmetricdibenzyl-substituted lactide).

It is important to note that the focus of the above-referenced ROPmethod is to synthesize polymers with high melting points, which iscontrary (i.e. the opposite of) the approach of the present invention(i.e., desired viscous polylactides). Further, Baker and Smith (2002)appear to only contemplate the use of metal organic catalysts; incontrast, the present invention can use solely an organic catalysts.

Their focus is herein on the synthesis and polymerization of thesymmetric dialkyl-substituted lactides, but the patent includes also thepossibility of polymerization of possible non-functionalmonoalkyl-substituted lactides. With emphasis on the synthesis of“poly(lactide)-based new materials with controlled properties” theinventors present here the synthesis and ROP of the mono-isopropyl (3),-butyl (4), -hexyl (5) and -benzyl (6) -substituted lactides, thedimethyl-substituted lactide (2) and the symmetric dihexyl-substitutedlactide (5′) (shown below), as well as their ring-openingpolymerizations with the “standard” FDA-approved (Food Drug Admin, 1975)metal organic catalysts Sn(Oct)₂, with the more reactive “tin-based”tin(II) trifluoromethane sulfonate Sn(OTf)₂ Möller et al., 2000; Mölleret al., 2001) and the solely organic catalyst 4-(dimethylamino)pyridineDMAP (Nederberg et al., 2001). The today accepted“coordination-insertion” mechanism for ROP of cyclic esters has beenwell established and described by Kowalski et al. (1998). Kricheldorf etal. (2000) and others for the most commonly used Sn(Oct)₂ catalyst. Inthis reaction mechanism Sn(Oct)₂ exchanges at least one of its2-ethylhexanoate ligands with the initiating alcohol to form a tinalkoxide initiator. After monomer ring-opening leading to an alcoholester end group the propagation proceeds through the tin alkoxide activecenters. This mechanism also applies to the ROP used here for thepolymerizations of the alkyl-substituted monomers catalyzed by Sn(Oct)₂,Sn(OTf)₂ and DMAP, respectively. The inventors utilized benzyl alcohol(BnOH) as the alcohol initiator, which enables later furtherfunctionalization of the polymers by deprotection of the benzyl endgroups with H₂/Pd. When the steric more hindered monomers could not beefficiently polymerized with Sn(Oct)₂ and Sn(OTf)₂, the use of the DMAPcatalyst was successfully applied. In most cases good control ofmolecular weight and narrow polydispersities were achieved for ROP ofthe new monoalkyl-substituted monomers leading to new functionalizedpoly(lactides).

Materials

α-hydroxyisobutyric acid, α-hydroxyisovaleric acid, 2-hydroxyoctanoicacid, D,L-3-phenyllactic acid, 2-bromopropionyl bromide were purchasedfrom Fluka (Buchs, Switzerland), 2-hydroxyhexanoic acid and2-bromopropionyl chloride from Sigma/Aldrich (Buchs, Switzerland).D,L-lactide from Purac Biochem (The Netherlands) was delivered undervacuum and directly transferred into a glove-box for storage. Tin(II)2-ethylhexanoate (Sn(Oct)₂) was purchased from Aldrich and used asreceived. Tin(II) trifluoromethane sulfonate (Sn(OTf)₂, Aldrich) and4-(dimethylamino)pyridine (DMAP, Fluka) were dried under vacuum at 80°C. prior to use. Benzyl alcohol (Fluka) was dried over calcium hydrideand distilled prior to use.

Solvents were dried by standard methods and distilled prior to use.Anhydrous pyridine (Fluka) was stored over molecular sieve.

Monomer Synthesis 3,6,6-Trimethyl-1,4-dioxane-2,5-dione (2) (referred asdimethyl-substituted lactide)

5 g α-hydroxyisobutyric acid (48 mmol) and 5.15 mL 2-bromopropionylchloride (50 mmol) were stirred at 75° C. under nitrogen for 12 h. 300mL acetone and 14 mL anhydrous triethylamine (100 mmol) were added tothe mixture and the solution was stirred for 3 h at 60° C. Afterfiltration of the triethylammonium chloride salts, acetone was distilledoff and the resulting mixture was dissolved in 450 mL ethylacetate:hexane mixture (1:1). After filtration over silica gel thesolvents were distilled off, and the remaining crude product wasrecrystallized from ethyl acetate:hexane mixture (1:10). ¹H NMR (500MHz, CDCl₃): δ 5.1 (q, 1H), 1.705 (s, 6H), 1.69 (d, 3H). ¹³C NMR (500MHz, CDCl₃): δ 168.58, 166.64, 80.54, 72.90, 26.23, 25.27, 17.41. ELEM.ANAL. Calcd. for C₇H₁₀O₄: C, 53.16; H, 6.33. Found: C, 52.86; H, 6.39.Yield: 46%.

3-Methyl-6-isopropyl-1,4-dioxane-2,5-dione (3) (isopropyl-substitutedlactide)

5.1 g α-hydroxyisovaleric acid (43 mmol) and 4.85 mL 2-bromopropionylbromide (45 mmol) were stirred at 75° C. under nitrogen for 12 h. 300 mLacetone and 12 mL anhydrous triethylamine (86 mmol) were added to themixture and the solution was stirred for 3 h at 60° C. After filtrationof the triethylammonium bromide salts, acetone was distilled off and theresulting mixture was dissolved in 500 mL ethyl acetate:hexane mixture(1:1). After filtration over silica gel the solvents were distilled off,and the remaining crude product was recrystallized from hexane. ¹H NMR(500 MHz, CDCl₃): δ 5.0 (q, 1H), 4.76 (d, 1H), 2.5 (m, 1H), 1.66 (d,3H), 1.16 (d, 3H), 1.06 (d, 3H). ¹³C NMR (500 MHz, CDCl₃): δ 167.59,166.16, 79.83, 72.10, 29.24, 18.51, 15.86, 15.79. ELEM. ANAL. Calcd. forC₈H₁₂O₄: C, 55.81; H, 6.98. Found: C, 55.55; H, 6.99. Yield: 35%.

3-Methyl-6-butyl-1,4-dioxane-2,5-dione (4) (butyl-substituted lactide)

2.5 g 2-hydroxyhexanoic acid (18.9 mmol) and 2.1 mL 2-bromopropionylbromide (19.7 mmol) were stirred at 75° C. under nitrogen for 12 h. 150mL acetone and 5.3 mL anhydrous triethylamine (38 mmol) were added tothe mixture and the solution was stirred for 3 h at 60° C. Afterfiltration of the triethylammonium bromide salts, acetone was distilledoff and the resulting mixture was dissolved in 300 mL ethylacetate:hexane mixture (1:2). After filtration over silica gel thesolvents were distilled off, and the remaining crude product wasrecrystallized from hexane. ¹H NMR (500 MHz, CDCl₃): δ 5.05 (q, 1H),4.90 (dd, 1H), 1.9-2.15 (br m, 2H), 1.68 (d), 1.65 (d), (3H, of 2diastereoisomers), 1.3-1.6 (br m, 4H), 0.93 (t, 3H). ¹³C NMR (500 MHz,CDCl₃): δ 167.54, 166.93, 166.25, 165.85, 75.79, 72.50, 72.24, 31.61,29.70, 26.69, 26.42, 22.19, 21.97, 17.52, 15.81, 13.72, 13.66. ELEM.ANAL. Calcd. for C₉H₁₄O₄: C, 58.06; H, 7.53. Found: C, 57.62; H, 7.60.Yield: 40%.

3-Methyl-6-hexyl-1,4-dioxane-2,5-dione (5) (hexyl-substituted lactide)

2.5 g 2-hydroxyoctanoic acid (15.6 mmol) and 1.75 mL 2-bromopropionylbromide (16.3 mmol) were stirred at 80° C. under nitrogen for 12 h. 150mL acetone and 4.35 mL anhydrous triethylamine (31 mmol) were added tothe mixture and the solution was stirred for 3 h at 60° C. Afterfiltration of the triethylammonium bromide salts, acetone was distilledoff and the resulting mixture was dissolved in 250 mL ethylacetate:hexane mixture (1:2). After filtration over silica gel thesolvents were distilled off, and the remaining crude product wasrecrystallized from hexane. ¹H NMR (500 MHz, CDCl₃): δ 5.00 (q, 1H),4.89 (dd, 1H), 1.9-2.15 (br m, 2H), 1.70 (d), 1.66 (d), (3H, of 2diastereoisomers), 1.45-1.65 (br m, 2H), 1.25-1.40 (br m, 6H), 0.90 (t,3H). ¹³C NMR (500 MHz, CDCl₃): δ 167.53, 166.90. 166.26, 165.87, 75.80,72.49, 72.25, 31.92, 31.45, 31.39, 30.00, 28.73, 28.50, 24.60, 24.29,22.48, 22.44, 17.54, 15.83, 14.00. ELEM. ANAL. Calcd. for C₁₁H₁₈O₄: C,61.68; H, 8.41. Found: C, 61.63; H, 8.48. Yield: 45%.

3-6-Dihexyl-1,4-dioxane-2,5-dione (5′) (symmetric hexyl-substitutedlactide)

A mixture of 2 g 2-hydroxyoctanoic acid (12.5 mmol) and 0.24 gp-toluenesulfonic acid (1.25 mmol) in 200 mL toluene was heated atreflux for 24 h, and the forming water removed continuously by using aDean-Stark aperture. The toluene was distilled off and the resultingmixture was dissolved in 200 mL ethyl acetate:hexane mixture (1:2) andfiltered over silica gel. After removal of the solvents the residue wasdissolved in diethylether. The solution was washed with sodium hydrogencarbonate (saturated solution) and dried over MgSO₄. The product wasrecrystallized from diethylether. ¹H NMR (500 MHz, CDCl₃): δ 4.90 (dd),4.87 (dd), (1H, of 2 diastereoisomers), 1.9-2.15 (br m, 2H), 1.4-1.6 (brm, 2H), 1.2-1.4 (br m, 6H), 0.88 (t, 3H). ¹³C NMR (500 MHz, CDCl₃): δ166.97, 165.83, 76.37, 75.58, 31.92, 31.37, 30.09, 28.70, 28.52, 24.47,24.29, 22.43, 13.94. Yield: 65%.

3-Benzyl-6-methyl-1,4-dioxane-2,5-dione (6) (benzyl-substituted lactide)

2.5 g D,L-3-phenyllactic acid (15 mmol) and 1.75 mL 2-bromopropionylbromide (16.3 mmol) were stirred at 90° C. under nitrogen for 12 h. 150mL acetone and 4.2 mL anhydrous triethylamine (30 mmol) were added tothe mixture and the solution was stirred for 3 h at 60° C. Afterfiltration of the salts, acetone was distilled off and the resultingmixture was dissolved in 250 mL ethyl acetate:hexane mixture (1:1).After filtration over silica gel the solvents were distilled off, andthe remaining crude product was recrystallized from 1:1 ethylacetate:hexane mixture. ¹H NMR (500 MHz, CDCl₃): δ 7.25-7.4 (m, 5H) □,5.30 (t) 5.10 (dd) (1H, of 2 diastereoisomers), 4.95 (q), 3.65 (q) (1Hof 2 diastereoisomers), 3.48 (dd), 3.37 (dd), 3.24 (dd) (2H, of 2diastereoisomers), 1.55 (d), 1.46 (d) (3H, of 2 diastereoisomers). ¹³CNMR (500 MHz, CDCl₃): δ 166.69, 166.07, 165.70, 165.50, 134.64, 130.03,129.85, 129.32, 128.71, 127.49, 77.94, 76.63, 72.43, 71.96, 38.93,36.36, 17.70, 16.01. ELEM. ANAL. Calcd. for C₁₂H₁₂O₄: C, 65.45; H, 5.45.Found: C, 65.34; H, 5.39. Yield: 35%.

2-(2-Bromo-1-oxopropoxy)octanoic acid (13)

2.5 g 2-hydroxyoctanoic acid (15.6 mmol) and 1.75 mL 2-bromopropionylbromide (16.3 mmol) were stirred at 75° C. under nitrogen for 12 h. Theobtained 2-(2-bromo-1-oxopropoxy)octanoic acid was purified by columnchromatography and characterized by NMR. ¹H NMR (500 MHz, CDCl₃): δ 5.08(dd, 1H), 4.47 (q, 1H), 1.9-2.0 (m, 2H), 1.88 (d), 1.85 (d), (3H, of 2diastereoisomers), 1.4-1.5 (m, 2H), 1.25-1.40 (br m, 6H), 0.90 (t, 3H).¹³C NMR (500 MHz, CDCl₃): δ 176.10, 169.90, 72.82, 39.07, 31.50, 30.78,28.60, 24.88, 22.47, 21.44, 14.00. Yield: 96%.

Preparation of Catalyst Stock Solutions

The catalysts for the ROP were used from stock solutions. A flask washeated under vacuum and after cooling to room temperature underprotecting gas placed into a glove-box. The catalysts were weighed inand the flasks sealed with a septum. Dry THF and toluene were addedafterwards under argon atmosphere. Stock solutions of Sn(Oct)₂ (intoluene/THF 3/2; 0.33 g/mL), Sn(OTf)₂ (in toluene/THF 1/1; 0.055 g/mL)and DMAP (in THF; 0.1 g/mL) were prepared.

General Procedure for Ring-Opening Polymerizations

Polymerizations were typically run with 2.3 mmol of monomer (˜0.4 g). Areaction flask containing a stirbar was fitted with a septum, flamedunder vacuum, and placed into a glove-box where the monomer was filled.The adequate solvent (2 mL) and catalyst (from stock solution, 1.5 mol %to monomer) were then added under argon atmosphere. After heating to asuitable temperature for solubilization of the reactants, benzyl alcoholas initiator alcohol was added as a 5-fold diluted solution in dry THF[40 μL, 0.076 mmol, for an expected degree of polymerization (DP) of30], and the mixture was heated to the desired polymerizationtemperature.

At the desired reaction time, the reactions were stopped by adding 2 mLof THF, followed by precipitation in hexane, hexane/diethylether (1/1)and cold methanol, respectively. In order to remove remaining catalystthe polymer was finally washed with methanol and dried at 40° C. undervacuum. Polymerization conversions and DP were determined by ¹H NMRanalysis, and molecular weights and polydispersities determined by GelPermeation Chromatography.

Measurements

The ¹H NMR spectra were recorded in either deuterated chloroform oracetone-d₆ with a Bruker spectrometer (500 MHz). Gel PermeationChromatography (GPC) was carried out on a Waters chromatographer,mounted with Styragel HR 1-4 columns (Waters) and connected to a Waters410 differential refractometer. THF was the continuous phase andpolystyrenes of known molecular weights: 500, 2630, 5970, 9100, 37900,96400 g/mol (Tosoh Corporation) were used as calibration standards.Glass transition temperatures (T_(g)) were measured with a differentialscanning calorimeter (SSC/5200, Seiko Instruments). Heating wasperformed under nitrogen at a flow rate of 5° C./min and the temperaturewas calibrated with an indium standard.

New Alkyl-Substituted Lactide Synthesis

The standard preparation method of symmetrical dialkyl-substitutedlactides is the condensation reaction of the α-hydroxy acid, generallyin acidic conditions and removing of the water formed during thereaction (Yin and Baker, 1999; Deane and Hammond, 1960). For thesynthesis of the unsymmetrical monoalkyl-substituted lactides,dimethyl-2, isopropyl-3, butyl-4, hexyl-5 and benzyl-6 substitutedlactide, the inventors chose a simple two-step one-pot synthesis, basedon the analogue reaction described by Schollkopf et al. (1979).Dialkyl-, and mixed substituted lactides can be prepared by the samemethod, by choosing accordingly the substituents R¹, R², R³ and R⁴, asoutlined above.

The α-hydroxy acid 7, bringing in the alkyl-substituent in the desiredlactide ring, leads under reaction (i) with an α-bromo alkanoyl bromide8 to the intermediate ester 9, as shown in Scheme 1. The ring closure tothe lactide 10 is obtained in a second step (ii) under basic reactionreactions conditions with triethylamine (Et₃N). This one-pot synthesishas proven to be quite versatile, being easily performed and based onrelative cheap starting materials. The proceeding of the reaction,quantitatively formation of the intermediate ester 9 along with thefully consumption of the initial α-hydroxy acid 7, can be monitored byThin Layer Chromatography (TLC). By this method the new isopropyl-3,butyl-4, and hexyl-6 substituted lactides were synthesized. To theinventors' knowledge these lactides have not been described in theliterature so far, but are mentioned as possible non-functionalalkyl-substituted lactides in the patent of Baker and Smith (2002). Themonobenzyl-substituted lactide 6 was synthesized by Schollkopf et al.before and Bolte et al. determined the crystal structure of thiscompound (Bolte et al., 1994), but both have not described anypolymerization reactions with this potential monomer. The synthesis ofthe dimethyl-substituted lactide 2 and its polymerization is reported byBaker and Smith (2002) and Chisholm et al. (2003). For the inventors'studies both substituted lactides 2 and 6 were prepared by the generaltwo-step one-pot method. The more steric-hindered dimethyl-substitutedlactide 2 was used here as a “difficult” polymerizable monomer for ingeneral comparison with the polymerization reactions of the newsubstituted lactides and for the investigation of ROP with Sn(Oct)₂, andthe more reactive catalysts Sn(OTf)₂ and DMAP, respectively. As atypical example for the synthesis of monoalkyl-substituted lactides theinventors describe here the synthesis of the monohexyl-substitutedlactide 5 in more detail. As illustrated in Scheme 2 in the firstreaction step (i) 2-hydroxyoctanoic acid 11 and 2-bromopropionyl bromide12 are simply mixed and stirred at 75° C. under nitrogen for 12 hforming the ester 2-(2-bromo-1-oxopropoxy)octanoic acid 13. Control byTLC (CH₂Cl₂:MeOH:CH₃COOH 10:0.1:0.1 eluent mixture, R_(f): 0.45) showedno remaining initial hydroxy acid reactant (R_(f): 0.15). The secondreaction step (ii), ring closure to the desired lactide, can be donedirectly in the same pot. Acetone is added for better solubility andunder the basic reaction conditions with triethylamine (Et₃N)3-methyl-6-hexyl-1,4-dioxane-2,5-dione 5 is formed while stirring for 3h at 60° C. Purification by filtration of the salts andrecrystallization from ethyl acetate:hexane mixtures yields the finallactide product 5, which after drying is ready for ring-openingpolymerization. In a further control experiment the ester 13 wasprepared by the same first reaction step, the reaction stopped after thesame time and after purification the ester2-(2-bromo-1-oxopropoxy)octanoic acid 13 was characterized by NMRproving its structure and quantitative formation. The yields obtainedfor the different alkyl-substituted lactides are of about 40-50%,showing the limiting step for this synthesis method is the formation ofthe lactide ring. Further optimization of the second reaction step (ii)of the presented one-pot synthesis could be achieved by applying anydissolution technique. Despite the two asymmetric α-carbon atoms in thelactide ring and the non-stereoselective reaction conditions the lactide3 was obtained as one major diastereomer, whereas the alkyl-substitutedlactides 4 and 5 were obtained in a 2:1 mixture, the benzyl-substitutedlactide 6 in a 8.5:1.5 mixture of diastereomers after the singlepurification step by recrystallization. For the initial studies ofring-opening polymerizations of these new lactides all monomers wereused in the described diastereomer mixtures. TLC tests of themonoalkyl-substituted lactides and the work-up by column chromatographyof the hexyl-substituted lactide 5 have shown that a separation of thelactide diastereomers is possible. This is of interest since it is knownthat the stereochemistry has a tremendous impact on the poly(lactide)properties. Thus this also applies for the overall tailoring of thematerial properties of the new poly(alkyl-substituted lactides) andtheir possible copolymers in all stereochemical combinations.

Ring-Opening Polymerizations

The controlled ROP of D,L-lactide in presence of metal catalysts is avery well known reaction. Especially Sn(Oct)₂ is an outstanding catalystfor ROP of lactides, not only because of its good polymerizationproperties, but also because of its FDA approval it has a majoradvantage for the synthesis of poly(lactides) for medical applications(Kricheldorf et al., 1995; Hyon et al., 1997; Schwach et al., 1997;Degee et al., 1999). The classical method is Sn(Oct)₂ catalyzedpolymerization of lactide in bulk, with or without an alcohol initiator,at relative high temperatures (>120° C.). Due to these reactionconditions side reactions, e.g. transesterifications, decrease thecontrol of molecular weight and polydispersity. For the controlledsynthesis of poly(lactides) in general and also for the inventors' newpoly(alkyl-substituted lactides) for biomedical applications theinventors intended to polymerize at lower reaction temperatures (<110°C.) in order to achieve best control of the polymer properties. Benzylalcohol was chosen as the initiator alcohol for different reasons: a)its low volatility for the standard reactions at around 100° C., b) withthe benzyl ¹H NMR signal at 7.4 ppm determination of conversions anddegrees of polymerization (DP) is possible for all the studiedalkyl-substituted monomers and polymers, and c) the H₂/Pd reduction ofthe resulting benzylester end group is giving reactive carboxylic acidend groups suitable for further functionalizations of the polymers. Forthe initial studies and comparisons of the different new monomers allpolymerizations were targeted for a degree of polymerization of DP=30,which also enables easy characterization via end group analysis by ¹HNMR. The Sn(Oct)₂ catalyst was used in relatively large amounts tofavour reasonable polymerization rates (1.5 mol % with respect to themonomer) but maintained in presence of excess of initiating alcohol([BnOH]/[Sn(Oct)₂]˜2, [M]/[BnOH]˜30), since it has been proven that thepolymerization rate is only dependent on the catalyst concentration,provided that [ROH]/[SnOct₂]>2 (Kowalski et al., 1998; Kricheldorf etal., 2000).

ROP of Monohexyl-Substituted Lactide 5 with Sn(Oct)₂

The synthesis of the dimethyl 2, isopropyl 3, butyl 4, hexyl 5 and 5′,and benzyl 6-substituted lactides was followed by ROP of these monomers.The different alkyl chain lengths of the substituents are expected toinfluence the hydrophobicity/philicity and degradability of the obtainedpolymers and thus to allow to modulate the poly(lactide) properties. Fora first feasibility study, the hexyl-substituted lactide 5 waspolymerized under standard reaction conditions with Sn(Oct)₂ incomparison with D,L-lactide 1. These results are presented in Table 1.The polymerization of the hexyl-substituted lactide 5 was first run at60° C., in solvent-free mild reaction conditions, taking advantage ofthe low melting point of this monomer. The conversion was determined by¹H NMR as described in FIGS. 2A-B on the crude reaction mixture, usingthe methine proton peak integrals of both polymer 14 and monomer 5.Since the conversion was only 13% after 1 hour and 25% after 5 h,polymerizations were then run at a higher temperature (100° C.). By thisa conversion of 83% was reached after 1 h. After 4 h reaction time theconversion increased to 90% and the molecular weight distribution wasstill narrow (M_(w)/M_(n)=1.13). However, a prolonged reaction time (24h) led to a broader distribution due to transesterification sidereactions (M_(w)/M_(n)=1.44). Replacing the methyl group of this monomerby another hexyl substituent (symmetric hexyl-substituted lactide 5′)did not significantly affect the polymerization rate since a similarconversion of 81% (instead of 83% for 5) was found after 1 h.Polymerizations with D,L-lactide 1 in similar conditions (in toluene at60° C. and in toluene/bulk at 100/110° C.) were carried out as controls.Conversions were determined by ¹H NMR analysis on the crude reactionmixture by using the methine peak integrals of both polymer and monomer(methine proton at 5.4 ppm for the monomer, 5.2 ppm for the polymer inacetone-d₆). The conversions were higher than those obtained for thehexyl-substituted monomer 5, particularly for low temperatures. Theseresults evidence that the steric hindrance of the hexyl group has adirect influence on the polymerization rate. It must be noticed that theMW distribution is narrower for the hexyl monomer (M_(w)/M_(n)=1.13)than for the D,L-lactide melt-polymerized (1.26). This might be due tothe low viscosity in the hexyl-substituted lactide reaction mixtureobserved during and still at the end of the ROP compared to theD,L-lactide polymerization, where a possible loss of polymerizationcontrol occurs near complete conversion. The obtained polymers showed DPvalues close to those expected from the monomer to initiator ratio(corrected with the conversion value), which is consistent with aliving-polymerization process.

TABLE 1 Time Conversion DP M_(n) Monomer ROP conditions (h) (¹H NMR)Targeted^(a) Measured^(b) (g/mol) M_(w)/M_(n) T_(g) (° C.) 5 Bulk 60° C.1 13 30 4 — — 5 Bulk 60° C. 5 25 30 8 — — 1 Tol. 60° C. 1 30 30 6 11601.16 1 Tol. 60° C. 5 63 30 20 3350 1.12 5 Bulk 100° C. 1 83 30 26 44501.13 5 Bulk 100° C. 5 90 30 29 4650 1.13 −17 5 Bulk 100° C. 24 92 30 304530 1.44 5′ Bulk 100° C. 1 81 28 22 5600 1.09 −47 1 Tol. 100° C. 1 9030 25 4020 1.07 +41 1 Bulk 110° C. 1 90 30 26 4000 1.26 ^(a)corrected DPtarget after determination by ¹H NMR on the crude polymer mixture.^(b)determined by ¹H NMR on the precipitated product.¹H NMR was used to confirm the structure of the poly(hexyl-substitutedlactide) 14 obtained after precipitation in MeOH. Both benzylester and—CH—OH end groups can be identified, confirming the initiation by thebenzyl alcohol. The methylene protons of the benzyl alcohol areoverlapped by the methine protons of the polymer chain at 5-5.25 ppm.The spectrum clearly showed that the alcohol end groups are of twotypes: a (q) CH3-CH—OH, 45% and b (dd) hexyl-CH—OH, 55% (calculated fromintegration). This result shows that the propagation proceeds by the“alcoholate” attack in nearly equal measure on both kinds of carbonylatoms in the substituted lactide ring with a slight preference for theless hindered one. This result is also consistent with the fact that asecond substitution of the remaining methyl group of 5 by another hexylgroup, giving the symmetrical hexyl-substituted lactide 5′, did notinduce a significant decrease of the conversion (81% and 83% for 5′ and5, respectively, after 1 h), under the same reaction conditions.

As demonstrated, the monohexyl-substituted lactide 5 could besuccessfully polymerized with Sn(Oct)₂ in relative high yields underconvenient reaction conditions (100° C., 1 h). Glass transitiontemperatures (T_(g)) were determined for the polylactides of D,L-lactide1, monohexyl-substituted lactide 5 and symmetric dihexyl-substitutedlactide 5′ with comparable molecular weights (4000-5500 g/mol) andmolecular weight distributions (M_(w)/M_(n)˜1.10). Interestingly, theglass transition temperature decreased from T_(g)=41° C. for standardpoly(D,L-lactide) to T_(g)=−17° C. for the poly(monohexyl-substitutedlactide) 14 and T_(g)=−47° C. for the poly(dihexyl-substituted lactide),showing that the physical properties of the polymers could be modulatedby the introduction of controlled amounts of hydrophobic moieties. T_(g)values measured for poly(D,L-lactide) and poly(dihexyl-substitutedlactide) were in a good accordance to those previously reported in theliterature: Jamshidi et al. (1988) obtained a T_(g)≈37° C. for apoly(D,L-lactide) of 3470 g/mol, and Yin and Baker (1999) reported aT_(g)≈−37° C. for their poly(dihexyl-substituted lactide). For thelatter value it is to point out that the molecular weight was muchhigher (43000 g/mol) than the one tested in the inventors' study (5600g/mol).

ROP of Monoalkyl-Substituted Lactides 2, 3, 4 and 6

With regards to the previous results on the monohexyl-substitutedlactide 5, Sn(Oct)₂-catalyzed polymerizations of the other dimethyl-2,isopropyl-3, butyl-4 and benzyl-6 substituted lactides were investigatedat 100° C. for 1 h, and the obtained results are presented in Table 2.Conversions were determined as described above by ¹H NMR analysis. Allmonomers could be polymerized with reasonable conversions (from 62 to87%) except for the dimethyl-substituted monomer 2 (22% even after 18h). All alkyl-substituted polymers showed relative narrow MWdistributions (M_(w)/M_(n)<1.2). Interestingly these MW distributionswere even narrower than those obtained for D,L-lactide 1. Molecularweights obtained and determined by GPC were in good accordance with theDP determined by ¹H NMR. Moreover the obtained DP were in relativelygood correlation with the aimed values (conversion*M/I) for thepolymers.

TABLE 2 ROP Time Conversion DP M_(n) Monomer Catalyst conditions (h) (¹HNMR) Targeted^(a) Measured^(b) (g/mol) M_(w)/M_(n) 1 Sn(Oct)₂ Bulk 110°C. 1 90 30 26 4000 1.26 2 ″ Bulk 100° C. 18 22 29 7 1120 1.29 3 ″ Bulk100° C. 1 62 26 16 2000 1.18 4 ″ Bulk 100° C. 1 87 26 20 3850 1.22 5 ″Bulk 100° C. 1 83 30 26 4450 1.13 6 ″ Tol. 100° C. 1 79 26 n.d.^(c) 31001.14 ^(a)corrected DP target after determination by ¹H NMR on the crudepolymer mixture. ^(b)determined by ¹H NMR on the precipitated product.^(c)not determinable by ¹H NMR from benzyl end groups, because ofoverlapping signals.

The found conversions were correlative with the chain length of thealkyl substituents of the monomers. The conversions decreased from 90 to87 and 83% with the increasing in the chain length of the methyl-1,butyl-4 and hexyl-5 substituent, respectively. Branchedalkyl-substituted monomers were less good polymerizable. The conversionof the isopropyl derivative 3 decreased to 62% under the same reactionconditions, and dramatically decreased to only 22% for the doublemethyl-substituted lactide 2 even after a prolonged reaction time of 18h. The benzylic derivative 6 was polymerizable with a good conversion of79% within 1 h. Due to the melting point of 6 (mp>100° C.), the ROP wascarried out in toluene. The steric hindrance by a benzyl or an isopropylgroup on the monomer did not compromise the polymerizability with theSn(Oct)₂ catalyst. In contrast the di-substitution of the α-carbon ofthe monomer by two methyl groups (dimethyl-substituted lactide 2) had adramatic negative effect on the polymerization rate. Baker and Smith(2002) reported the difficulty to polymerize this monomer with Sn(Oct)₂.They had to use a very high polymerization temperature (180° C.) toobtain a decent conversion (75%) after 24 h. Except for this particularcase of the disubstituted monomer 2, Sn(Oct)₂ has been proven to berelatively efficient in polymerizing the differently steric-hinderedmonomers (conversions of 65-85%) in short reaction times. Neverthelessmore reactive catalysts were investigated, on the one hand to favorhigher conversion rates for the more hindered monomers, as the dimethyl2 and isopropyl 3 ones, and on the other hand to achieve even higheryields for the other new monomers at mild polymerization conditions, andalso with a view to the potential scale-up of the reaction.

ROP with Sn(OTf)₂ and DMAP Catalysts

Sn(OTf)₂ was previously reported by Möller et al. (2000); Möller et al.,2001) as a very efficient and versatile catalyst for ROP of variouslactides and lactones, compared to other tin-based catalysts as Sn(Oct)₂or dibutyltin(II)-2-ethylhexanoate Bu₂Sn(Oct)₂. Polymerizations werecarried out with the Sn(OTf)₂ catalyst for the more hindered dimethylsubstituted lactide 2 under the same conditions as those used for theSn(Oct)₂ polymerizations (1.5 mol % of catalyst to monomer, 100° C.). Asa control polymerizations with the D,L-lactide 1 were performed. Theresults are shown in Table 3. The triflate catalyst showed a slightincrease in the polymerization rate of the dimethyl-substituted lactide2 in the reactions either in bulk or in pyridine (30% and 27%,respectively, after 18 h), compared to Sn(Oct)₂ (22% conversion after 18h, Table 2). Pyridine was used since it was shown that Sn(OTf)₂ is avery efficient catalyst for ROP in this solvent. Indeed, for theD,L-lactide control reactions the conversion is only about 30% after 1.5h with Sn(Oct)₂ as catalyst, whereas it is 70% with Sn(OTf)₂ (Table 3).However, bulk or toluene conditions are not preferable here for thetin-triflate catalyst, since only 77% and 10% conversion are obtainedafter 1.5 h reaction time, in comparison to 90% conversion with Sn(Oct)₂in both conditions after 1 h (Table 1). This might be due to the poorsolubility of the Sn(OTf)₂ catalyst in molten mixture or in toluene,hence the typical conditions the inventors used for the polymerizationsat 100° C., in bulk or toluene, are not really adequate for thiscatalyst to show its efficiency. Therefore the catalyst was changed fromSn(OTf)₂ to 4-(dimethylamino)pyridine (DMAP), which was recentlyreported by Nederberg et al. (2001) as an efficient organic catalyst forlactide polymerization. Same bulk polymerization conditions as usedbefore for Sn(Oct)₂ were applied. Used in the same concentration asSn(Oct)₂ and Sn(OTf)₂ (1.5%/monomer i.e BnOH/DMAP=2), the DMAP catalystwas found to be more reactive (Table 4). The conversion for thedimethyl-substituted lactide 2 was already 30% after only 5 h ofpolymerization, whereas it was 22% with Sn(Oct)₂ (Table 2) and 30% withSn(OTf)₂ (Table 3) after a reaction time of 18 h. After 24 h theconversion reached 65%, showing a good control of molecular weight anddistribution (M_(w)/M_(n)=1.26). In comparison the D,L-lactide 1conversion is already 78% after only 0.6 h. To enhance thepolymerization rate further polymerizations were run with a higheramount of DMAP (2 equivalents of DMAP/BnOH), a catalyst concentrationtypically used in previous studies Nederberg et al. (2001). As a resultthe conversion for 2 was then 35% after only 1 h. Also the otheralkyl-substituted lactides of both linear type, hexyl-substitutedlactide 5, and non-linear type, isopropyl- and benzyl-substitutedlactides 3 and 6, were polymerized with this higher catalystconcentration for DMAP. The conversions reached now excellent 80%, 97%,and 95% for 3, 5 and 6 after a reaction time of only 1 h. Polymers ofhigher DP could be obtained (e.g. hexyl-substituted lactide 5, DP=41),requiring however a longer reaction time for good conversion (2 h and 5h for 59% and 90% conversion, respectively). This can be explained bythe higher viscosity of the reaction mixture observed at conversionsabove 60%. DMAP is also efficient at a lower polymerization temperature.60% conversion could be obtained within 18 h for the benzyl-substitutedlactide 6 by carrying out the polymerization at 60° C. in THF. This isan interesting result, since this solvent was generally reported to benot preferable for ROP of lactides with tin-based catalysts (Simmons andBaker, 2001).

TABLE 3 ROP Time Conversion DP M_(n) Monomer Catalyst conditions (h) (¹HNMR) Targeted^(a) Measured^(b) (g/mol) M_(w)/M_(n) 2 Sn(OTf)₂ Bulk 100°C. 18 30 26 6 850 1.20 2 ″ Pyr. 100° C. 3 0 26 — — — 2 ″ Pyr. 100° C. 1827 26 6 850 1.25 1 ″ Bulk 110° C. 1.5 77 30 25 4200 1.22 1 ″ Pyr. 100°C. 1.5 70 30 18 2900 1.23 1 Sn(Oct)₂ Pyr. 100° C. 1.5 30 30 n.d^(c)—^(c) —^(c) 1 Sn(OTf)₂ Tol. 100° C. 1.5 10 30 —^(c) —^(c) —^(c)^(a)corrected DP target after determination by ¹H NMR on the crudepolymer mixture. ^(b)determined by ¹H NMR on the precipitated product.^(c)not determined after 1.5 h reaction time, polymerization wascontinued.

TABLE 4 BnOH/DMAP ROP Time Conversion DP M_(n) Monomer ratio conditions(h) (¹H NMR) Targeted^(a) Measured^(b) (g/mol) M_(w)/M_(n) 2 2 Bulk 100°C. 5 30 27  8 1410 1.19 2 2 ″ 24 65 27 21 2600 1.26 2 0.5 ″ 1 35 27n.d.^(c) —^(c) —^(c) 2 0.25 ″ 5 75 27 22 2100 1.35 3 0.5 Bulk 100° C. 180 26 20 3450 1.15 5 0.5 Bulk 100° C. 1 97 29 24 5400 1.10 5 0.5 ″ 2 5945 26 5400 1.09 5 0.5 ″ 5 90 45 41 7600 1.19 6 0.5 Bulk 100° C. 1 95 27n.d.^(d) 3000 1.20 6 0.5 THF 60° C. 18 60 27 n.d.^(d) 2600 1.20 1 2 Bulk110° C. 0.6 78 31 26 4050 1.18 1 0.5 ″ 0.6 92 30 25 3200 1.36 1 0.5 ″ 199 30 31 3900 1.48 ^(a)corrected DP target after determination by ¹H NMRon the crude polymer mixture. ^(b)determined by ¹H NMR on theprecipitated product. ^(c)not determined after 1.5 h reaction time,polymerization was continued ^(d)not determinable by ¹H NMR from benzylend groups, because of overlapping signals.

Again the molecular weight distribution was much narrower for thesubstituted lactides (M_(w)/M_(n)=1.10-1.20) than for D,L-lactide(1.48). Since D,L-lactide is the more reactive monomer, probably moretransesterification side reactions occur to the end of thepolymerization at high conversion. However, these undesirable sidereactions were reported not to occur even after a prolonged time ofpolymerization (no polydispersity increase) with the DMAP catalyst atlow temperatures (35° C.) (Nederberg et al., 2001). Thus DMAPpolymerization could be run to completion at milder reaction conditionsto obtain optimal yields. DP and M_(n) obtained for the new substitutedlactides were close to those expected from the theory, consistent with agood polymerization control. The increased concentration of DMAP (4 eq.to BnOH) allowed further enhancement of the polymerization rate. Here aconversion of 75% could be obtained for the dimethyl-substituted monomer2 within only 5 h. However, in this last case the molecular weight islower than the desired one and the polydispersity increases from 1.26 to1.35. Further studies on optimization of these DMAP-catalyzed ROP ofalkyl-substituted lactides will be carried out. Finally it is to pointout that the DMAP catalyst was efficiently removed from the polymersduring the precipitation in MeOH, no traces of DMAP were observed on ¹HNMR polymer spectra. Therefore this organic catalyst appears to be apromising versatile catalyst for ROP of steric-hindered and substitutedlactides by enabling polymerizations at mild temperatures and thusfavoring the synthesis of narrowly dispersed new poly(alkyl-substitutedlactides).

The inventors report here a versatile approach for the synthesis and thepolymerization of new monoalkyl-substituted lactide monomers. The newmonomers can easily be obtained by a two-step-one-pot synthesis, and canbe polymerized by ROP with Sn(Oct)₂, Sn(OTf)₂ and DMAP as catalysts. Bythe introduction of alkyl substituents the polymerizability of themonomers was as expected in accordance with their steric hindrance.Nevertheless Sn(Oct)₂ was applied successfully for the ROP of thedifferent substituted lactides at 100° C., and all polymers are showingnarrow molecular weight distributions (M_(w)/M_(n)=1.1-1.2) andconversions of 65-85% within 1 hour. Only the dimethyl-substitutedlactide 2 was hardly polymerizable even after prolonged reaction times(22% conversion after 18 h). The polymerizations rates of thealkyl-substituted lactides could be increased by using either Sn(OTf)₂or DMAP as ROP catalysts. Particularly with the latter one a conversionof about 70% could be reached for the dimethyl-substituted lactide 2within in a decent polymerization time of 24 h, and conversions of about90% in 1 h were obtained for the other substituted new lactides. Theefficiency of this catalyst for ROP opens doors to the potential designof new functionalized lactides in a large variety, in respect to thepossible variations of the alkyl substitutents R¹, R², R³ and R⁴,respectively in the two basic starting compounds 7 and 8 (Scheme 1).These new lactide based monomers and reactive ROP catalysts are apromising approach for the controlled synthesis of tailored materialsand its applications e.g. in the medical field. By adjusting differentparameters such as the substituents on the lactide monomers, polymermolecular weight or the combination and incorporation in copolymers withestablished poly(lactides) and poly(lactide-co-glycolides) importantproperties like polymer hydrophilicity/-phobicity and biodegradabilitycan be suited to the specific application.

Example 2 Synthesis and Properties of Novel Poly(Hexyl-SubstitutedLactides) for Pharmaceutical Applications

Biocompatible and biodegradable polylactides/glycolides (PLA/PLGA) havereceived high attention over the last thirty years in the biomedicalfield as sutures, implants, colloidal drug delivery systems (Penning etal., 1993; Uhrich et al., 1999), and more recently also in tissuerepairing and engineering (Liu and Ma, 2004; Stock and Mayer, 2001) andanti-cancer drug delivery (Mu and Feng, 2003; Jiang et al., 2005). Nextto the medical field they are also widely used in the packaging area. Asbiodegradable “green polymers” they are preferable to the commoditypolymers currently used (Drumright et al., 2000; Vink et al., 2003).There is a crucial need of well-defined polylactide-based materials withadvanced properties to fit all the requirements for the differentapplications. For example, PLA/PLGA homo- and co-polymers synthesized bythe well-established ring opening polymerization (ROP) process(Dechy-Cabaret et al., 2004; Kricheldorf et al., 1995; Schwach et al.,1997; Degee et al., 1999; Ryner et al., 2001) have a glass transitiontemperature (T_(g)) limited to a range of only 40-60° C. (Jamshidi etal., 1988; Vert et al., 1984), independent of the polymer molecularweight and chemical composition. This combined with interestingmechanical properties makes them suitable in medical applications asbiodegradable implants, bone fracture fixation devices, scaffolds forliving cells. However for drug delivery purposes, they need to beformulated with organic solvents and administered as solutions or inform of nano- and micro-particles, they can not be injected on theirown. This strategy for novel PLAs with tailored properties is based onthe substitution of a methyl ligand on the lactide monomer by otheralkyl substituents (Trimaille et al., 2004). The introduction ofalkyl-side groups is expected to strongly affect material propertiessuch as the T_(g) and viscosity. For pharmaceutical applications furtherimportant properties such as degradation rate and profile or drugencapsulation and release will be modified.

Increasing attention was recently put on injectable polymers aspromising alternatives to emulsions, liposomes or microsphere drugdelivery systems (Amsden et al., 2004; Hatefi and Amsden, 2002; Merkliet al., 1994). Next to other alkyl-substituted lactides the inventorsreported the synthesis and characterization of the novelpoly(monohexyl-substituted lactide) (PmHLA), which was shown to have alow glass transition temperature (T_(g)=−17° C.) compared to a standardPLA (Tg=41° C.) with the analogue molecular weight (4500 g/mol)(Trimaille et al., 2004). Based on these initial results thishydrophobic hexyl-substituted polylactide could be favourable andinteresting for applications as an injectable PLA drug delivery systemcomparable to the reported semi-solid hydrophobic poly(ortho esters)(Schwach-Abdellaoui et al., 2001). The used ring-opening polymerizationtechnique with its living character and functional end groups gives theopportunity to synthesize various new PLA-based copolymers incombination with the established PLA/PLGA systems. By this differentfunctional PLAs with tailored material properties for biomedicalapplications can be easily obtained.

The inventors present here a detailed study on the synthesis andcontrolled ROP of these novel hexyl-substituted polylactides, as well astheir physico-chemical properties in terms of T_(g) and rheologicalbehaviour and degradation kinetics and mechanism.

Materials

All materials here were prepared as described in EXAMPLE 1

Monomer Synthesis

All synthesized monomers were prepared as described in EXAMPLE 1

Polymer Synthesis and Characterization

All synthesized polymers were prepared and characterized as described inEXAMPLE 1

Thermal Analysis

Glass transition temperatures (T_(g)) were measured with a differentialscanning calorimeter (SSC/5200, Seiko Instruments). Heating wasperformed at a flow rate of 5° C./min and the temperature was calibratedwith an indium standard.

Viscosity Determination

Viscosities were determined using a Bohlin controlled stress rheometerwith a parallel plate PU 20 device (Bohlin Rheology GmbH, Mühlacker,Germany). A stress viscosity test (rotation) was applied to the sampleswhich were placed on the stationary lower plate. The temperature wasfixed at 25° C. or 37° C. during the test with a Bohlin ExtendedTemperature Option (ETO). Shear rates ranging from 0.1 to 400 s⁻¹ wereused for determination. For all samples an integration time of 20 s anda delay time of 20 s were used.

Degradation Studies

40 mg of polymer were placed into flasks and gently heated to be abovethe T_(g) of the polymers. 5 mL of 0.1M phosphate buffer pH 7.4 werethen added and the flasks slowly agitated at the adequate temperature.At predetermined times polymers were collected, rinsed with milli-Qwater and dried to constant weight prior to determination of mass lossand average molecular weight.

Mass loss (ML %) was evaluated by gravimetric analysis and calculatedfrom:

${M\; L\mspace{14mu} \%} = \frac{100( {W_{0} - W_{t}} )}{W_{0}}$

Where W₀ and W_(t) are the initial weight and residual weight of the drypolymer at time t.

Molecular weights were determined by GPC by dissolving the polymer inTHF as described above.

Monomer Synthesis and Controlled ROP

In the inventors' previous work the inventors reported the synthesis andring-opening polymerization of novel alkyl-substituted lactide monomersfor the design of new tailored polylactide materials (Trimaille et al.,2004). Here the inventors focus on the poly(monohexyl-substitutedlactide) (PmHLA 5) which was obtained by the synthesis pathway presentedin Scheme 3. The synthesis of the new monohexyl-substituted lactide(mHLA 4) is based on a “two step one pot” reaction of 2-hydroxyoctanoicacid 2, easily synthesized in large scale from heptanal 1, with2-bromopropionyl bromide leading to an intermediate ester 3, whichundergoes ring-closing after changing to basic reaction conditions withtriethylamine. This latter intramolecular cyclization is found to be thelimiting step of the process with a yield of 45%, despite the dropwiseaddition of the intermediate 3 into the very dilute basic solution tofavor the ring-closing. After recrystallization the mHLA was obtained asa mixture of the two diastereomers (ratio 2/1) which can be easilyseparated by gel chromatography. This offers interesting perspectivesfor the properties of the hexyl-substituted polylactides obtained fromthe diastereomerically pure monomers by ROP, considering the greatimpact of the stereochemistry on the physico-chemical characteristics ofthe polymer materials (Tsuji et al., 1991). In the present work theinventors investigated the diastereomeric mixture of the monomer forcomparison of the obtained polymer properties with those of theamorphous poly(D,L-lactide).

ROP of the mHLA were carried out in convenient bulk conditions at 100°C. with two catalysts, Sn(Oct)₂ or DMAP (respectively 1 and 2 eq. to theinitiator), in the presence of benzyl alcohol as initiator. Benzylalcohol was chosen for its suitability to be cleaved off with H₂/Pd andsetting free the reactive carboxylic acid end group on the PLA polymerchain for possible further functionalizations. The degree ofpolymerization (DP), which can be controlled by adjusting the ratio of[monomer]/[BnOH], was targeted at 45 to yield a polymer of about 8000g/mol molecular weight. Predictable molecular weights and narrowpolydispersities were obtained for both catalysts (Table 5). For thesame polymerization time of 4 hours, the conversion obtained forSn(Oct)₂ catalyst (95%) was slightly higher than that observed for DMAP(82%), even when this latter was used in higher amounts up to 2 eq. toinitiator. Thus the classical Sn(Oct)₂ catalyst was selected for thePmHLA synthesis, having the further advantage of being already FDAapproved (Food Drug Admin. Food, 1975). Studies on the ROP kinetics ofthe mHLA with this catalyst were performed in terms of molecular weightversus conversion (FIG. 1). For a targeted DP of 45, a linear functionwas obtained showing that the polymerization is well controlled and of a“living character”. A typical ¹H NMR spectrum of the PmHLA afterpurification by precipitation in methanol is presented in FIG. 3 and isconsistent with this “living character” of the ROP with the presence ofboth signals of the benzylester and CHOH end groups, which confirms theinitiation of the ROP by the alcoholate active species. Same studieswere performed with the DMAP catalyst and showed the same “living”behaviour. For a high DP of 120, a loss in the control began to occurfor conversions higher than 50% (FIG. 1). This is probably due to theincreasing viscosity of the bulk reaction mixture, increasingtransesterification side reactions rather than the polymer chain growthvia the active polymer end group.

TABLE 5 ROP of mHLA with Sn(Oct)₂ and DMAP at 100° C. in bulk. DP M_(n)Catalyst Catalyst/BnOH Time (h) Conversion Targeted Measured^(a) (g/mol)M_(w)/M_(n) Sn(Oct)₂ 1 4 95 45 39 7500 1.25 DMAP 2 4 82 45 35 7100 1.15^(a)Determined by ¹H NMR on the precipitated polymer using signals ofthe benzyl protons of the benzyl ester end groups

In FIGS. 2A-B the actual and expected molecular weights for differenttargeted DPs are reported.

The obtained molecular weights correspond to the theoretically expectedones up to a DP=60. For higher molecular weights a loss in the controlof polymerization of mHLA (a) compared to that of D,L-lactide (b)appears. Here a better control can be achieved by changing the reactionsconditions. However for the purpose of the desired pharmaceuticalapplications, this range of controlled polymer molecular weight from2500 to 10000 g/mol (e.g. DP=10 to 50) is quite satisfactory sincerather low viscosities and reasonable degradation times will be requiredfor the potential use as injectable systems. Further investigationsPmHLAs of different DPs up to 60 (M_(n) from 2800 to 9100 g/mol) werethen prepared with very good control and quite narrow polydispersities(M_(w)/M_(n)˜1.19-1.35) in bulk conditions at 100° C. (Table 6) bytargeting the adequate DP and adjusting the polymerization times. Forthe PmHLA of the lowest M_(n) of 2800 g/mol, the highest polydispersitywas observed (M_(w)/M_(n)=1.35). Indeed the conversion was rapidlycomplete for this low targeted DP, and then side transesterificationreactions began to occur. Here the polymerization time can be shortenedto improve the molecular weight distribution. Standard PLA of about 7500g/mol was prepared under comparable reaction conditions as a control. Apolymerization time of 1.5 hours was sufficient to obtain a conversionof about 90% whereas longer times were required for ROP of thehexyl-substituted lactide due to the steric hindrance of the hexyl sidegroups, e.g. 2 hours for the analog PmHLA of 7500 g/mol.

TABLE 6 Synthesis of PmHLA of different molecular weights (ROP at 100°C.) and their material properties. ROP Time DP M_(n) M_(w) T_(g) η₀ [25°C.] Monomer solvent (h) Targeted Measured^(a) (g/mol) (g/mol)M_(w)/M_(n) (° C.) (Pa · s) mHLA Bulk 1.5 15 13 2800 3750 1.35 −22.4 140mHLA Bulk 1.5 30 26 4800 6000 1.25 −17 715 mHLA Bulk 2 36 33 6500 77001.19 −13.3 1750 mHLA Bulk 2 45 40 7500 9400 1.25 −12 3500 mHLA Bulk 3 6059 9100 11850 1.30 −10.5 4850 D,L-LA Tol. 1.5 50 48 7200 9200 1.28 40glassy ^(a)Determined by ¹H NMR on the precipitated polymer usingsignals of the benzyl protons of the benzyl ester end groups

Polymer Physical Properties

Polymer material properties were particularly investigated in terms ofglass transition temperatures (T_(g)) and melt viscosity, two importantparameters for the evaluation of the “injectability” of the material.The T_(g) varied from −22.5 to −10° C. in the range of the investigatedmolecular weights (Table 6). The evolution of the T_(g) as a function ofM_(n) fits quite well the Fox-Flory equation, as shown in FIG. 3 by theplot of T_(g) as a function of the reciprocal M_(n). For the same M_(n)of about 7500, PmHLA and PLA presented radically different T_(g) valueswith −12° C. for PmHLA and 40° C. for PLA. The impact of the flexiblehexyl groups on the glass transition temperature and other physicalproperties in comparison to standard PLA are obvious. Due to their lowT_(g) the poly(hexyl-substituted lactides) are in a rubber viscous stateat room temperature. For envisioned medical applications by injectionthe viscosity for these polymers can be controlled by choosing theappropriate molecular weight. In Table 6 the zero shear viscosity values(e.g. in the Newtonian domain) at 25° C. are presented. Here for all themolecular weights PmHLA typically behaved like a Newtonian fluid for ashear rate ranging from 0.1 to 10 s⁻¹ with a constant viscosity. A shearthinning behaviour was observed above this value as it is known for manypolymers. The zero shear viscosity varied from 140 to 4850 Pa·s at 25°C. and from 45 to 720 Pa·s at 37° C. by increasing the molecular weightM_(n) from 2800 to 9100 g/mol. As shown in FIG. 4, the variation of thePmHLA zero shear viscosity with M_(w) (in log-log scale) followed theFox and Loshaek (1955) theory with a coefficient slope α≈3.2 independentof the temperature used, suggesting an entanglement point M_(c) of thepolymer inferior to M_(w)=3700 g/mol and an interpenetration of thePmHLA chains (Porter and Johnson, 1966). In conclusion the physicalproperties or the “injectability” of the hexyl substituted polylactidescan be modulated and fine-tuned by varying the molecular weight of thesepolylactides. Moreover they can be very good predicted from the alreadywell-established calculation models.

Degradation Studies

For the purpose of injectable drug delivery systems the degradability ofthe new poly(hexyl-substituted lactide) was investigated. Due to thehexyl side groups this new polylactide is much more hydrophobic than acomparable standard PLA and causes differences to the degradation ofPLA. The degradation mechanism of PmHLA and PLA of comparable molecularweights was investigated in terms of molecular weight and weight loss asa function of time in phosphate buffer pH 7.4 at 37° C. The results areshown in FIG. 6B for PmHLA and PLA of comparable molecular weights of7500 g/mol. The molecular weight decrease profile for PmHLA was similarto that of standard PLA. The hexyl groups on the PmHLA could be expectedto decrease the rate of hydrolysis of the ester bonds, because of theirhigher steric hindrance and a possible hydrophobic protection againstwater and its hydroxyl ions. But in fact the degradation rate wasslightly higher for the PmHLA, which can be explained by the physicalstate of the polymers at 37° C. Here PLA is a glassy rigid polymer(T_(g)=40° C.) whereas PmHLA is in a rubber viscous state (T_(g)=−12°C.). The latter state favours the penetration of the water into thepolymer matrix, leading to a higher hydrolyzation rate, what is alsoreported by Ye et al. (1997). This influence of the physical state ofthe polymer on the degradation profile was confirmed by furtherdegradation studies performed at 60° C., a temperature which is a abovethe T_(g) of both polymers. As expected the degradation rates stronglyincreased, but now PmHLA degraded slower than PLA. At 60° C. PmHLA andPLA are both in a rubber state, and thus the slower degradation forPmHLA could only be attributed to the presence and influence of thehydrophobic hexyl groups on the polymer.

Following up the degradation over the whole time period of seven weeksthe degradation process could be divided into two phases. In the firstweeks of degradation no mass loss was observed and the decrease inmolecular weight was the most pronounced. In the second phase the onsetof the mass loss occurred together with a slower decrease of themolecular weight. This is typical for a “bulk erosion” mechanism, inwhich the hydrolysis first occurs in the inner polymer matrix with arandom scission of the ester bonds due to water absorption, followed bythe diffusion of the small oligomers formed out of the polymer bulk.This was corroborated by the visual aspect of the polymer, which gotswollen in the first phase of degradation due to the water absorption.Whereas the “bulk erosion” mechanism is well-known for thepoly(D,L-lactide) (Hakkarainen et al., 1996), this mechanism was quitesurprising for the hydrophobic PmHLA. Due to the strong hydrophobiceffect of the hexyl side groups along the polymer backbone one couldexpect a “surface erosion” mechanism, as it is known for the hydrophobicpoly(ortho-esters). In fact, for the studied PmHLAs themono-substitution pattern and the existing number of hexyl side groupson the polylactide backbone do not have a strong enough hydrophobiceffect of being water repellent and protecting the inner polymer matrixagainst hydrolyzation. Finally, it is to point out that despite bothpolymers had a similar degradation profile in terms of molecular weightdecrease, the weight loss of PLA occurred earlier than that of PmHLA.This effect might most probably be due to the less water-solublehexyl-substituted degradation residues from the PmHLA, which diffuseless good from the inner polymer matrix into the outer aqueous phase.

As the inventors demonstrated the viscosity of this novelmonohexyl-substituted PLA can be tailored in order to obtain injectablepolymers which at the same time show a nice degradability profile forcontrolled drug release. Investigations with different incorporateddrugs are in progress and will be presented in the near future.

The inventors have presented in this Example the controlled synthesisand properties of a novel poly(monohexyl-substituted lactide) (PmHLA).The ROP of the mHLA monomer for targeted DPs up to 60 was wellcontrolled with a living character as shown by molecular weight versusconversion studies and ¹H NMR. The hexyl side groups along thePLA-polymer backbone had a strong impact on the physical properties interms of glass transition temperature (T_(g)) and viscosity. Thesevalues can be easily tailored by adjusting the polymers molecularweight, corresponding with the Fox and Flory laws. The poly(monohexyl-substituted lactide) (PmHLA) shows low viscosity and is suitablefor injectable drug delivery systems. Since these new hydrophobicpolylactide based polymers are synthesized by ring-openingpolymerization (ROP) a fine-tuning of this parameter can possibly beachieved by adjusting the number of hexyl side groups choosing theappropriate ratio of monohexyl lactide- and D,L-lactide monomer for theresulting copolymers. Moreover the functional end groups of these newpolymers can be used for adding and building up advanced molecularstructures with further functionalities needed for optimized drugdelivery systems.

Under physiological conditions the degradation mechanism of PmHLA can bedescribed as “bulk erosion” and the degradation rate is similar to thatof standard PLA. Further investigations on differentpoly(alkyl-substituted lactides), also in combination of copolymers withestablished PLA/PLGA polymers in various compositions and macromoleculararchitectures are also envisioned.

Example 3 Controlled Drug Release Using Novel Poly(Hexyl-SubstitutedLactides)

Biocompatible and biodegradable polylactides/glycolides (PLA/PLGA) havereceived great attention over the last thirty years in the biomedicalfield as sutures, implants, colloidal drug delivery systems, (Penning etal., 1993; Uhrich et al., 1999, in tissue repairing and engineering (Liuand Ma, 2004; Stock and Mayer, Jr., 2001), and also in anti-cancer drugdelivery (Mu and Feng, 2003; Jiang et al., 2005). Next to the medicalfield they are used and of high interest in many other applications e.g.in the packaging area as environmentally friendly materials compared tothe commodity polymers currently used (Drumright et al., 2000; Vink etal., 2003). Composting of the waste of PLA derived products leads bydegradation back to the non toxic natural lactic acid, which is on theone hand the starting material for these polymers and on the other handadvantageously obtained from renewable resources such as corn starch.Despite the possibility of producing PLA and its copolymers ofcontrolled molecular weight and chemical composition from thelactide/glycolide monomers by ring-opening polymerization (ROP)(Dechy-Cabaret et al., 2004; Kricheldorf et al., 1995; Schwach et al.,1997; Degee et al., 1999; Ryner et al., 2001), these materials do notalways show suitable or optimal properties for all the desiredapplications. For example the PLA/PLGA polymers usually have a glasstransition temperature of around 40-60° C. (Jamshidi et al., 1988; Vertet al., 1984), and therefore are not applicable on their own forinjectable drug delivery systems. Injectable polymers themselves arereceiving increased attention (Amsden et al., 2004; Hatefi and Amsden,2002), as alternatives to emulsions, liposomes or microsphere injectabledrug delivery systems.

The inventors reported in a previous paper the synthesis andring-opening polymerization (ROP) of new monoalkyl substituted lactides,with a view to design new controlled tailored polymeric PLA basedmaterials (Trimaille et al., 2004). The strategy of substitution of themethyl ligand of the lactide monomer by an other substituent and thereofthe synthesis of new functionalized polylactide-based materials by ROPcan be achieved by using new reactive catalysts, e.g. tin(II)trifluoromethane sulfonate and 4-dimethylaminopyridine (Trimaille etal., 2004; Moller et al., 2001; Nederberg et al., 2001). These catalystsfacilitate the ROP of very steric hindered monomers with a good controlof molecular weight and molecular weight distribution. Next to otheralkyl-substituted lactides the inventors described the synthesis andcharacterization of new polylactides obtained from monohexyl-substitutedlactide and dihexyl-substituted lactide [poly(monohexyl-substitutedlactide) and poly(dihexyl-substituted lactide)] which were shown to havelow glass transition temperatures (T_(g)<−15° C.) compared to a standardPLA with the analogue molecular weight (Trimaille et al., 2004). Basedon these initial results the new hydrophobic substituted polylactidescould be favourable and interesting for applications as injectablesemi-solid materials for drug delivery comparable to the reportedhydrophobic poly(ortho esters) (Merkli et al., 1994; Schwach-Abdellaouiet al., 2001). Moreover, the increased hydrophobicity of thesePLA-polymers could lead to further interesting properties in colloidalsystems, e.g. in terms of encapsulation efficiency and release ofhydrophobic drugs. The inventors' initial studies showed thatnanoparticles can be obtained from these new polymers by thenanoprecipitation technique. The used ring-opening polymerizationtechnique with its living character and functional end groups also givesthe opportunity to synthesize various new PLA-based copolymers incombination with the established PLA-PLGA systems. By this an enlargedlibrary of different functional PLAs with tailored material propertiesfor biomedical applications can be easily obtained.

The inventors present in this Example a detailed study on these newhexyl-substituted polylactides, their physico-chemical properties,degradation kinetics and mechanism, identification of the degradationproducts, and release of tetracycline as a model drug, all in comparisonto the analogue standard PLA.

Materials:

All materials here were prepared as described in EXAMPLE 1

Monomer Synthesis:

All synthesized monomers were prepared as described in EXAMPLE 1

Polymer Synthesis and Characterization:

All synthesized polymers were prepared and characterized as described inEXAMPLE 1 and EXAMPLE 2, respectively.

Degradation Studies

40 mg of polymer were placed into flasks and gently heated above theT_(g) of the polymers. 5 mL of 0.1M phosphate buffer pH 7.4 were thenadded and the flasks were slowly agitated at the adequate temperature.At predetermined times polymers were collected, rinsed with milli-Qwater and dried to constant weight prior to determination of mass lossand average molecular weight.

Mass loss (ML %) was evaluated by gravimetric analysis and calculatedby:

${M\; L\mspace{14mu} \%} = \frac{100( {W_{0} - W_{t}} )}{W_{0}}$

W₀ and W_(t) are the initial weight and the residual weight of the drypolymer at time t.

The molecular weights were determined by GPC as described above.

Electrospray Ionization/Mass Spectrometry.

The degradation products were identified by ESI/MS analysis, which wasperformed on a Finnigan MAT SSQ7000 quadripole mass spectrometer. Theresidues present in the aqueous degradation medium, were dried undervacuum, dissolved in a CH₂Cl₂/MeOH 9/1 mixture and 150 μL/min wereinfused into the ESI part. A voltage of 4500 V was maintained on theESI/MS electrode in order to form the multiprotonated ions of eachproduct. Mass was scanned from 50 to 1500 mass/charge units and 20 scansin 1 min were averaged to obtain the mass spectra.

Tetracycline Release Studies

200 mg polymer and 20 mg tetracycline hydrochloride (TH) were dissolvedin 1.7 mL THF and 1 mL methanol and well mixed. The mixture was dried invacuo at 35° C. for 72 h. From this 30 mg of the tetracycline loadedpolymer samples were incubated with 10 mL phosphate buffer 0.1 M pH=7.4at 37° C. The releasing medium was replaced every day with fresh buffersolution and stored at 4° C. before analysis. The amount of drugreleased was determined by using a HPLC system with a pump (Waters 600Econtroller), an autoinjector (Waters 717 plus autosampler), a UVdetector (Waters 2487) and an integrator (Millenium software, Waters).The column used was Nucleosil 100-5 C18 (Macherey-Nagel® Gmbh & Co.,Düren, Germany) with 5 μm particle size, 250 mm length and 4 mm innerdiameter. The mobile phase was a mixture of Milli-Q water andacetonitrile (81.5/18.5 v/v) containing 0.03 M EDTA, 0.011 M KNO₃ andacetic acid to give pH 3. A flow rate of 0.7 mL/min was used. Thesolution was degassed with helium prior to use. Standard solutions of THof concentrations ranging from 5 to 50 μs/mL in phosphate buffer 0.1 MpH 7.4 were prepared for calibration. The typical retention time of THwas 15.7 min monitored at 353 nm.

Polymer Synthesis and Properties

Based on a quite versatile synthesis method new alkyl-substitutedlactide monomers were synthesized for further ring-openingpolymerization. The synthesis steps of the here investigatedpoly(monohexyl-(PmHLA 6) and dihexyl-(PdiHLA 7) substituted lactides)are presented in Scheme 4. The synthesis of the newmonohexyl-substituted lactide (mHLA) 4 is based on a “two step one pot”reaction of 2-hydroxyoctanoic acid 2 with 2-bromopropionyl bromideleading to an intermediate ester 3, which undergoes ring-closing afterchanging to basic reaction conditions with triethylamine. Thedihexyl-substituted lactide (diHLA) 5 was synthesized by the simplecondensation reaction of the 2-hydroxyoctanoic 2 acid withp-toluenesulfonic acid. The 2-hydroxyoctanoic acid 2 was easilysynthesized in large scale from heptanal 1 (Shiosaki and Rapoport,1985).

Ring-opening polymerizations of mHLA, diHLA and D,L-lactide wereperformed with the FDA-approved Sn(Oct)₂ catalyst (Food Drug Admin.(1975) and benzyl alcohol as an initiator. Benzyl alcohol was chosen forits suitability to be cleaved off with H₂/Pd and setting free thereactive carboxylic acid end group on the PLA polymer chain for possiblefurther functionalizations. The molecular weight and degree ofpolymerization (DP) can be controlled by adjusting the ratio of[monomer]/[BnOH]. Polymerizations of the hexyl-substituted lactidescould be run in solvent-free conditions at a relative moderatetemperature of 100° C. due to their low melting point. In contrast forthe D,L-lactide polymerizations a solvent is necessary at this reactiontemperature and ideally toluene was used.

For the potential use as injectable drug delivery systems poly(hexyl-substituted lactides) with rather low viscosities, reasonabledegradation times and drug release rates were desired. The favourableviscosities are given for these polymers having a molecular weight of2000 to 10000 g/mol or in degree of polymerization DP=10 to 50. Thesepolymers could be prepared with very good control and narrowpolydispersities by ring-opening polymerization in bulk conditions. Asshown in FIGS. 6A-B the obtained molecular weights correspond to thetheoretically expected one up to a DP=60. For higher molecular weights aloss in the control of polymerization of mHLA (a) compared to that ofD,L-lactide (b) appears, due to the increasing viscosity of the bulkreaction mixture leading to a loss in monomer reactivity. Here a bettercontrol can be achieved by changing the reactions conditions. Thepolymers selected for the inventors' investigations and their materialproperties are summarized in Table 7 and discussed in the following.PmHLA and PLA of same molecular weights of about 4500, 7500 and 9000g/mol were prepared with quite narrow polydispersities by targeting theadequate DP and adjusting the polymerization times. A PdiHLA of 4500g/mol was synthesized as a further comparison for studying the influenceof the higher number of hexyl substituents along the PLA backbone.Polymerization times of 1.5 hours were sufficient to obtain D,L-lactideROP conversions of about 90% whereas longer times were required for ROPof the substituted lactides due to the steric hindrance of the hexylside groups (1.5 to 4 hours depending on the DP and the nature of themonomer). The glass transition temperatures (T_(g)) ranged from −17 to−10° C. for PmHLA and from 38 to 41° C. for standard PLA by varying thepolymer molecular weights from 4500 to 9100 g/mol. The impact of theflexible hexyl groups on the glass transition temperature and otherphysical properties in comparison to standard PLA are obvious. The T_(g)could be further decreased by increasing the number of hexyl groups onthe polymer as shown for PdiHLA of 4500 g/mol with a T_(g) of −42° C.,which is 25° C. less than for the analogue monohexyl-substitutedlactide. Due to their low T_(g) the poly(hexyl-substituted lactides) arein a rubber viscous state at room temperature. For envisioned medicalapplications by injection the viscosity for these polymers can becontrolled by choosing the appropriate molecular weight, as shown inFIG. 5. Also tailoring the number of hexyl side groups along the polymerbackbone e.g. in random copolymers with PLA are possible. PmHLAtypically behaved like a Newtonian fluid for a shear rate ranging from0.1 to 10 s⁻¹, and a shear thinning behaviour was observed above thisvalue, as it is known for many polymers. Looking at the Newtonian domainthe viscosity (η₀) varied from 140 to 4850 Pa·s at 25° C. and from 45 to720 Pa·s at 37° C. by increasing the molecular weight M_(n) from 2800 to9100 g/mol. PdiHLA shows the same behaviour as PmHLA under shear rate,but the Newtonian viscosity with 40 Pa·s was significantly lower thanthat observed for PmHLA with 715 Pa·s at 25° C. for a M_(n) of about4500 g/mol (see Table 7). This can be explained by the increasingflexibility of the whole polymer brought by the hydrophobic and highernumber of hexyl side groups. In conclusion the physical properties orthe “injectability” of the hexyl-substituted polylactides can bemodulated and fine-tuned by varying the molecular weight or the numberof hexyl groups on these polylactides.

TABLE 7 Reaction conditions (100° C.) and characteristics of PLA, PmHLAand PdiHLA. ROP Time DP M_(n) M_(w) T_(g) η₀ 25° C. Monomer solvent (h)Targeted Measured (g/mol) (g/mol) M_(w)/M_(n) (° C.) (Pa · s) D,L-LATol. 1.5 35 34 4820 6510 1.35 38 glassy D,L-LA Tol. 1.5 50 48 7200 92001.28 40 glassy D,L-LA Tol. 1.5 70 68 9000 11800 1.31 41 glassy mHLA Bulk1.5 30 28 4800 6000 1.25 −17 715 mHLA Bulk 2 45 41 7500 9000 1.20 −123500 mHLA Bulk 3 60 59 9100 12070 1.32 −10.5 4850 diHLA Bulk 4 25 204500 5620 1.25 −42 40

Degradation Studies

For the purpose of injectable drug delivery systems the degradability ofthe new poly(hexyl-substituted lactides) was investigated. Due to thehexyl side groups these new polylactides are much more hydrophobic thana comparable standard PLA and cause differences to the degradation ofPLA. Moreover the degradation rate and degradation mechanism hasimportant influence on the drug release profile. In the following theinventors present the results of the degradation studies of PmHLA andPLA at physiological conditions.

The degradation mechanism of PmHLA and PLA of comparable molecularweights was investigated in terms of molecular weight and weight loss asa function of time in phosphate buffer pH 7.4 at 37° C. The results areshown in FIGS. 6A-C. Independent of the chosen initial molecular weightof the polymers, the molecular weight decrease profile for PmHLA wassimilar to that of standard PLA. The hexyl groups on the PmHLA could beexpected to decrease the rate of hydrolysis of the ester bonds, becauseof their higher steric hindrance and a possible hydrophobic protectionagainst water and its hydroxyl ions. But in fact the degradation ratewas slightly higher for the PmHLA, which can be explained by thephysical state of the polymers at 37° C. Here PLA is a glassy rigidpolymer (T_(g)=40° C.) whereas PmHLA is in a rubber viscous state(T_(g)˜−15° C.). The latter state favours the penetration of the waterinto the polymer matrix, what is also reported by Ye et al. (1997)leading to a higher hydrolyzation rate.

Due to the ROP in the presence of benzyl alcohol as initiator, thesynthesized polylactides are bearing benzyl ester end groups. PmHLA wasalternatively synthesized with water as initiator for obtaining theanalogue polylactides with carboxyl end groups for investigating theimpact of these acid groups on the degradation rate. As shown in FIG. 6Ano difference was observed for the degradation profile of the benzylester terminated and the carboxyl terminated PmHLAs.

Following up the degradation over the whole time period of seven weeksthe degradation process could be divided into two phases. In the firstweeks of degradation no mass loss was observed and the decrease inmolecular weight was fast independent of the starting molecular weight.In the second phase the onset of the mass loss occurred together with aslower decrease of the molecular weight. This is typical for a “bulkerosion” mechanism, in which the hydrolysis first occurs in the innerpolymer matrix with a random scission of the ester bonds due to waterabsorption, followed by the diffusion of the small oligomers formed outof the polymer bulk. This was corroborated by the visual aspect of thepolymer, which got swollen in the first phase of degradation due to thewater absorption. This “bulk erosion” mechanism is well-known for thepoly(D,L-lactide) (Hakkarainen et al., 1996; Grizzi et al., 1995). Dueto a possible strong hydrophobic effect of the hexyl side groups alongthe polymer backbone also a “surface erosion” mechanism could beconsidered as known for the hydrophobic poly(ortho-esters) (Gurny etal., 1999). In conclusion for the studied PmHLAs the mono-substitutionpattern and the existing number of hexyl side groups on the polylactidebackbone do not have a strong enough hydrophobic effect of being waterrepellent and protecting the inner polymer matrix against hydrolyzation.

The degradation studies showed further that with the higher molecularweight the polymers have a later onset of weight loss (FIGS. 6A-C).Despite both polymers had a similar degradation profile in terms ofmolecular weight decrease, the weight loss of PLA occurred earlier thanthat of PmHLA. This effect might most probably be due to the lesswater-soluble hexyl-substituted degradation residues from the PmHLA,which diffuse less good from the inner polymer matrix into the outeraqueous phase.

Further degradation studies were performed at 60° C., a temperaturewhich is above the T_(g) of both polymers (FIG. 7). As expected thedegradation rates strongly increased, but now PmHLA degraded slower thanPLA. This result confirms the above discussed influence of the physicalstate of the polymer on the degradation profile. At 60° C. PmHLA and PLAare both in a rubber state, and now the slower degradation for PmHLA canbe attributed to the presence and influence of the hydrophobic hexylgroups on the polymer. This becomes even more evident when comparing thedegradation of PmHLA with the doubled numbered hexyl-substituted PdiHLAof the same M_(n) of about 4500 g/mol, as shown in FIG. 6A. Bothpolymers are in a rubber state and the molecular weight decrease wasmuch slower for the PdiHLA with a time of latency at the beginning and amolecular weight plateau at 75% of the initial M_(n) compared to aplateau at 45% for PmHLA. PdiHLA showed hardly degradability, neither athigher temperatures.

The identification of the degradation products is crucial in theperspective to toxicity issues for a potential use of thesehexyl-substituted polylactides as injectable drug delivery systems inthe human body. Degradation compounds present in the aqueous phase wereanalyzed by electrospray ionization/mass spectrometry (EIS/MS). Thespectrum of PmHLA residues after 48 hours of degradation at 85° C. wasdetermined. The higher temperature was chosen to accelerate thedegradation. All molecular weight peaks were identified as beingoligo-esters of different sizes arising from the hydrolysis of the esterbonds of the PmHLA (Table 8). The spectrum obtained after 60 hoursdegradation time revealed only the presence of the molecular weightpeaks of 2-hydroxyoctanoic acid 2 (M=159) and lactic acid (M=89). Bothcompounds were used initially for the synthesis of these polymers,whereby lactic acid is as a natural compound present in the human bodynon toxic and 2-hydroxyoctanoic acid is approved by the FDA for topicalapplications (Hall and Hill, 1986).

TABLE 8 Structures and molecular weight assignment for the degradationproducts of PmHLA. I

M = 90 II

M = 160 III

M = 214n + 18 IV

M = 214n + 90 V

M = 214n + 160 structure Peak peak + 1^(a) I   89  90 II  159  160 III n= 1  231  232 III n = 2  445  446 III n = 3  659  660 III n = 4  873 874 III n = 5 1087 1088 IV n = 1  303  304 IV n = 2  517  518 IV n = 3 731  732 IV n = 4  945  946 V n = 1  373  374 V n = 2  587  588 V n = 3 801  802 V n = 4 1015 1016 V n = 5 1229 1230 ^(a)Actual molecularweights are peak molecular weights plus atomic weight of one proton(negative polarization).

Tetracycline Release

Tetracycline hydrochloride (TH) and tetracycline free base (TB) weretested as a model drug for release studies from the hexyl-substitutedpolylactides. This broad-spectrum antibiotic has been extensively usedin human medicine. Recently Heller and Gurny successfully applied it inthe hydrophobic semi-solid poly(ortho esters) (POE) for injectabledrug-delivery systems for the treatment of periodontal diseases(Schwach-Abdellaoui et al., 2001a; Schwach-Abdellaoui et al., 2001b).Here investigations with tetracycline hydrochloride (TH) shouldfacilitate easy comparison with the inventors' previous data from thePOE studies and check on similarities and differences for these twokinds of hydrophobic polymers.

TH-loaded PmHLA (10% w/w) was simply prepared by dissolution of thepolymer and the drug in THF/methanol followed by drying under vacuum.PmHLA with the molecular weight of M_(n)=4500 g/mol was selected,because of its low viscosity and potential use as an injectable drugdelivery system. Tetracycline-loaded PLA of the same molecular weightwas prepared for a control. GPC analysis was performed afterincorporation of tetracycline to check on the initial stability of thedrug loaded polymers. Molecular weights remained constant at 37° C., nopolymer degradation could be observed possibly induced by the acidiccharacter of the TH. The release profile of the tetracycline from bothPmHLA and PLA is shown in FIG. 8. The release of TH was faster fromPmHLA than from PLA, and much higher in case of both polymers in thefirst 6 days than in the days afterwards. Interestingly the total amountof released TH was higher from PmHLA (45% of the initial TH loaded) thanfrom the standard PLA (23%), although the polymers had the samedegradation rate (FIG. 6A). The inventors attribute this to the easierdiffusion of TH from the PmHLA bulk matrix to the outer phase and intothe aqueous degradation medium, because of the rubber viscous state ofPmHLA compared to the rigid PLA at 37° C. Comparable release studieswere also performed with tetracycline free base (TB). The amount ofreleased TB was again higher from PmHLA than from PLA. For both polymersthe amounts of released TB were much lower than in the case of TH. Thetetracycline free base (TB) is much less water-soluble than thecorresponding hydrochloride (TH) thus the diffusion with water from theinner polymer matrix to the outer phase is slower. The total amount ofthe initial loaded TH in the polymers was not entirely recovered afterrelease. In fact it could be shown that the TH was partly degraded intothe less active epimer 4-epitetracycline. In all the HPLC analyses ofthe releasing media the signal of this by-product was found besides theactive TH. It was previously reported that TH degraded quickly in acidicconditions leading to about 50% of the epimer form (Schwach-Abdellaouiet al., 2001). Therefore it is probable that also here TH is degradedinside the polymer matrix, which has an acidic environment while polymerdegradation takes place. The released proportion of the medical activeTH compared to 4-epitetracycline was systematically higher for PmHLAthan for the standard PLA.

In conclusion the TH drug release from PmHLA with its higher rate andhigher total amount of active TH is favourable to PLA. Interestingly theTH release profile from PmHLA is with 45% better than that reported forcommercial Actisite fibers, which is loaded with 25% TH and releasesabout 30% active TH within 14 days (Schwach-Abdellaoui et al., 2001.

In this Example, the inventors present the controlled synthesis andproperties of new poly(hexyl-substituted lactides) (PHLA). The hexylside groups along the PLA-polymer backbone have a strong impact on thephysical properties in terms of glass transition temperature (T_(g)) andviscosity. These values can be easily tailored by adjusting the polymersmolecular weight. The presented poly(mono hexyl-substituted lactide)(PmHLA) shows low viscosity and is suitable for injectable drug deliverysystems. Since these new hydrophobic polylactide based polymers aresynthesized by ring-opening polymerization (ROP) a fine-tuning of thisparameter can possibly be achieved by adjusting the number of hexyl sidegroups choosing the appropriate ratio of monohexyl lactide andD,L-lactide monomer for the resulting copolymers. Moreover thefunctional end groups of these new polymers can be used for adding andbuilding up advanced molecular structures with further functionalitiesneeded for optimized drug delivery systems.

Under physiological conditions the degradation mechanism of PmHLA can bedescribed as “bulk erosion” and the degradation rate is similar to thatof standard PLA. The initial release studies of tetracyclinehydrochloride (TH) and tetracycline free base (TB) as model drugs showedan even better release of the active form of tetracycline from PmHLAthan from the analogue PLA.

Further investigations on different poly(alkyl-substituted lactides),also in combination of copolymers with established PLA/PLGA polymers invarious compositions and macromolecular architectures are currently inprogress. The development and optimization of these hydrophobicsubstituted polylactide based polymers for different medicalapplications will be presented in the near future.

Example 4 Novel Amphiphilic Methoxy Poly(EthyleneGlycol)-Poly(Hexyl-Substituted Lactide) Block Copolymers as HydrophobicDrug Carriers

This Example presents novel amphiphilic methoxy-poly(ethyleneglycol)-poly(hexyl-substituted lactides) block copolymers which weresynthesized by ring-opening polymerization (ROP) of mono anddihexyl-substituted lactide (mHLA and diHLA) in bulk at 100° C. in thepresence of tin(II) 2-ethylhexanoate (Sn(Oct)₂) as catalyst andmethoxy-poly(ethylene glycol) (MPEG) as initiator. MPEG-PmHLA andMPEG-PdiHLA copolymers of predictable molecular weights and narrowpolydispersities were obtained, as shown by ¹H NMR and GPC. DSCexperiments showed that the MPEG-PHLA block-copolymer presents a bulkmicrostructure containing MPEG domains segregated from the PHLA domains.Micelles were successfully prepared from these block copolymers, withsizes ranging from 30 to 80 nm. As expected, the critical micellarconcentration (CMC) is found to decrease with the increasing of numberof hexyl groups on the polyester block (MPEG-PLA>MPEG-PmHLA>MPEG-PdiHLA)for copolymers of the same composition and molecular weight, allowing toenvision these micelles as drug carriers in dilute conditions. Theincreased hydrophobicity of the micelle core by the higher number ofhexyl groups on the PLA chain was evidenced by Nile Red absorbanceexperiments, with higher amounts of the dye incorporated in themicelles. These novel amphiphilic copolymers in micelle state are of agreat interest for optimized hydrophobic drug loadings, as it was shownwith the griseofulvin model drug.

Due to their safe and biodegradable properties, amphiphilic PEG-PLA orPEG-PLGA block copolymers have been extensively studied over the pastfew decades as drug carriers (Kataoka et al., 2001; Yasugi et al., 1999;Riley et al., 2001; Lin et al., 2003), particularly recently for thedelivery of anti-cancer drugs (Yoo and Park, 2001; Zhang et al., 2005).Such di-block copolymers can self-assemble in aqueous medium to formspherical micelles (˜50 nm) with a core formed by the hydrophobicpolylactide segments and a surrounding shell consisting of thehydrophilic PEG chains. The latter stabilizes the surfaces in aqueoussystems and ensures a long half-life in the blood compartment due to thereduced interaction with the biological components (Gref et al., 1995).Many potential hydrophobic drugs can be easily entrapped in the core ofthese micelles, but drug loadings are often low and need to be improvedto be efficient for medical applications. The inventors recentlydescribed the potential of novel poly(hexyl-substituted lactides) PHLAas an alternative to standard PLA with regards to drug release,degradability and injectability (Trimaille et al., 2004; also see aboveExamples); The use of these hydrophobic alkyl-substituted PLA with PEGcan be interesting for drug delivery due to their micelle size, drugloading and degradability. The inventors describe here the synthesis andcharacterization of these novel amphiphilic block copolymers, as well asthe preparation and properties of the micelles in terms of encapsulationcapacity of a hydrophobic drug (griseofulvin). The results are discussedand compared with standard PLA-PEG di-block copolymers, of samemolecular weights and composition.

Materials:

All materials here were prepared as described in EXAMPLE 1.

Monomer Synthesis:

All synthesized monomers were prepared as described in EXAMPLE 1.

Polymer Synthesis and Characterization:

All synthesized polymers were prepared and characterized as described inthe above EXAMPLES.

Micelle Preparation and Characterization

20 mg purified copolymer and different amounts of GF were dissolved in 2mL acetone. The solution was then added dropwise (1 droplet/3 s) in 4 mLmilli-Q water under stirring. The acetone and a part of the water wereremoved under reduced pressure to reach a typical micelle concentrationof 5.2 mg/mL.

The mean size of the micelles was determined by quasi-elastic lightscattering (QELS) with a scattering angle of 90° C. at 25° C. using aMalvern Zetasizer 3000HS (UK), equipped with a He—Ne laser (633 nm).

Nile Red absorbance experiments on blank micelles (no GF): 20 μL of astock solution of Nile Red (0.97 mg/mL of THF/acetone 1/2) were added to1.5 mL of micelle solutions from each copolymer (MPEG-PLA, -PmHLA,-PdiHLA of same M_(n) and composition) of a given concentration (2.1mg/mL). A PEG solution (2.1 mg/mL water), and pure water (instead ofmicelle solutions) were also incubated with Nile red, as references Thesolutions were equilibrated in the darkness for overnight, with removalof THF and acetone. UV-visible spectra of each solution were recordedfrom 450-650 nm and corrected with the corresponding blanks preparedwithout Nile red.

Critical micellar concentration (CMC) determination (blank micelles):Fluorescence measurements were performed with Nile Red to determine theCMC. Different dilutions were prepared from the 5 mg/mL stock solutionof micelles to obtain samples of concentration ranging from 0.0001 to 1mg/mL. Then 2 μL of a Nile Red stock solution in acetone (0.97 mg/mL)were added to 200 μL of each sample, and the acetone was evaporated.Fluorescence measurements were performed using a Safire (Tecan)microplate reader in a 96-well plate. Emission spectra were recordedfrom 560 to 750 nm using a λ_(exc)=550 nm. The CMC was determined at theinflection point on the plots representing the maximum emissionwavelength as a function of the copolymer concentration, as previouslydescribed by Coutinho et al. (2002)

Measurements of GF Levels by HPLC

The micellar solutions were centrifuged at 6000 g for 7 minutes toremove the non-entrapped GF. Then 100 μL were dissolved in 900 μLacetonitrile to destroy the micelles and assay the GF which wasencapsulated. The HPLC system consisted in a pump (Waters 600Econtroller), an autoinjector (Waters 717 plus autosampler), a UVdetector (Waters 2487) and an integrator (Millenium software, Waters).The column used was Nucleosil 100-5 C18 (Macherey-Nagel® Gmbh & Co.,Düren, Germany) with 5 μm particle size, 250 mm length and 4 mm innerdiameter. The mobile phase was a mixture of 45 mM potassium dihydrogenphosphate solution (in Milli-Q water) and acetonitrile (45 v %).Pyrophosphoric acid was used to give pH 3. A flow rate of 1 mL/min wasused. The solution was degassed with helium prior to use. Standardsolutions of GF in water/acetonitrile (1/9) of concentrations rangingfrom 2 to 18 μg/mL were prepared for calibration. The typical retentiontime of GF was 9.5 min monitored at 293 nm. It was checked thatPEG-P(H)LA copolymers caused no interference at this wavelength.

Synthesis and Characterization of Amphiphilic PEG-PHLA.

In the inventors' previous work the inventors reported the synthesis andring-opening polymerization of novel alkyl-substituted lactide monomersfor the design of functionalized polylactide materials (Trimaille etal., 2004). The inventors have particularly focused on the mono and dihexyl-substituted lactide (mHLA and diHLA, respectively), which ledafter ring-opening polymerization to new biodegradable polymers withinteresting physical properties in comparison to standard PLA/PLGA.Here, studies are presented on the potential of these hydrophobicpolylactides in combination with PEG as novel amphiphilic blockcopolymers. The mHLA synthesis is based on a “two step one pot” reactionof 2-hydroxyoctanoic acid 2, easily synthesized in large scale fromheptanal 1, with 2-bromopropionyl bromide leading to an intermediateester 3, which undergoes ring-closing after changing to basic reactionconditions with triethylamine (Scheme 5). The diHLA 5 was synthesized bythe simple condensation reaction of the 2-hydroxyoctanoic acid withp-toluenesulfonic acid in a Dean-Stark apparatus. The di-blockMPEG-PmHLA 6 and MPEG-PdiHLA 7 copolymers were synthesized byring-opening polymerization of the mHLA and diHLA, respectively, in bulkat 100° C. using methoxy-PEG-OH 2000 g/mol (DP˜45, referred as MPEG2) asan initiator and the FDA-approved catalyst Sn(Oct)₂ (molar ratioSn(Oct)₂/MPEG=0.5), as described in Scheme 5. Both initiator andcatalyst were used as stock solutions in THF, and the solvent beingremoved from the reaction flask in the beginning of the polymerization.D,L-lactide was polymerized under the same conditions as a reference.

The results of the ROP of the different lactides are presented in Table9. First, ROP was performed with a targeted DP of 15 for both mHLA andD,L-lactide. 1 hour polymerization time appeared sufficient to reach anearly complete conversion for D,L-lactide (95%). The conversionobtained for mHLA after the same polymerization time was a bit lower(84%, data not shown), due to the steric hindrance brought by the hexylside groups. Therefore the ROP of mHLA was performed for 1.5 hours andthe conversion was nearly complete (94%). A polymerization time of 1.5hours was also sufficient to reach an acceptable conversion of the mHLAfor a targeted DP of 30 (90%). A prolonged time of 4 hours was requiredto obtain a good conversion (>90%) for the even more hindered diHLA. Inthe following, the copolymers will be referred as MPEG2-PLA3,MPEG2-PHLA3 and MPEG2-PHLA5, respectively (MPEGx-P(H)LAy, where x and yrepresent the M_(n) of the MPEG and P(H)LA blocks in kg/mol,respectively).

TABLE 9 Characteristics of the copolymers obtained by ROP of thedifferent lactides with MPEG 2000 g/mol as initiator (bulk, 100° C.)PEG/PHLA Time Lactide DP Lactide (wt %) M_(n) Name Monomer (h) Targ. expconv. (%) Feed Exp. (g/mol) M_(w)/M_(n) MPEG2-PLA3 D,L-LA 1 15 15 9548/52 42/58 4800 1.18 MPEG2-PmHLA3 mHLA 1.5 15 13 94 38/62 39/61 50501.18 MPEG2-PmHLA5 mHLA 1.5 30 27 90 24/76 28/72 7100 1.17 MPEG2-PdiHLA3diHLA 4 20 12 >90 26/74 38/62 4750 1.29

All copolymer molecular weights were close to those expected, withnarrow polydispersities (M_(w)/M_(n)˜1.15), indicating that thepolymerizations were well controlled. The shift in retention timeobserved by GPC analysis for MPEG-PmHLA compared to MPEG showed that theMPEG “macro-alcohol” initiated the polymerization well. As an examplefor the polymer characterization, the ¹H-NMR spectrum (300 MHz, CDCl₃)was determined for the MPEG-PHLA copolymer after purification byprecipitation in hexane. Both MPEG and lactide proton peaks could beclearly identified, and the composition of the copolymer could bededuced from the peak integrals of the CH₂ protons of the PEG and themethine protons of the hexyl lactide. ¹H-NMR spectrum of MPEG2-PmHLA3was also determined. Compositions were close to those expected (Table9), confirming that the ROP were well controlled.

Analysis of the Copolymer Microstructure by DSC

DSC experiments were performed on MPEG-PHLA copolymers to elucidate thecopolymer microstructure and the results are shown in FIG. 9 for thecase of MPEG-PmHLA copolymers. As a reference, both PmHLA5 and MPEG2homopolymers were analyzed. PmHLA5 is an amorphous polymer showing a lowT_(g)=−16° C., whereas MPEG2 is highly crystalline, showing a meltingpeak T_(m)=56° C. The DSC spectra of the MPEG-PHLA block-copolymersshowed characteristic features of both homopolymers, demonstrating thatthe copolymer present a bulk microstructure containing MPEG domainssegregated from PmHLA domains. However the shift in both melting andglass transition temperatures, compared to the homopolymers, show thatthere are interactions between both polymer chains. They can beattributed to their covalent attachment, limiting the mobility of theMPEG as well as the PmHLA chains. When increasing the chain length ofthe PmHLA in the block copolymer, the melting temperature shifted tolower values (from 56° C. to 36° C.), together with a decrease in thepeak intensity. This demonstrates a less pronounced crystallinity in theblock copolymer in comparison to the MPEG homopolymer. The T_(g) of thecopolymers (−12.5 and −13° C.) was slightly increased compared to theone of PmHLA homopolymer (−16.2° C.), which also is probably due to areduced mobility of the PmHLA segments when incorporated in thecopolymer, as discussed before.

DSC analysis for the MPEG2-PdiHLA3 copolymer showed typically the sameprofile as MPEG-PmHLA with a T_(m) of 38° C. and a T_(g) of −42° C. (theT_(g) of the PdiHLA homopolymer of M_(n)=5600 g/mol is −47° C., aspreviously reported (Trimaille et al., 2004).

Micelle Preparation and Characterization

The amphiphilic nature of the diblock copolymers, consisting of ahydrophobic P(H)LA and a hydrophilic PEG segment, provides theopportunity to form micelles in water with a PLA core and a PEG shell.MPEG2-PLA3, MPEG2-PLA3, MPEG2-PLA3 copolymers, with the same molecularweight (˜5000 g/mol) and composition (PEG/PLA˜40/60 in wt %) were usedfor the micelle preparation. The polymeric micelles were prepared by adirect precipitation method using acetone (Yoo and Park, 2001). Thelatter was removed under reduced pressure after the micelle preparation.The mean size of the micelles was determined by QELS measurements at 90°C. and the results are presented in Table 10 (column 0 mg/g). Thesmallest micelles were observed for the MPEG2-PdiHLA3 copolymer with 30nm mean diameter compared to about 70 nm for the analogs MPEG2-PLA3 andMPEG2-PmHLA3. This can be explained by the higher hydrophobicity of thepolylactide block when increasing the number of hexyl groups along thechain, favouring a stronger shrinkage upon addition of the water, as thenon-solvent, during the micelle preparation. It is to point out that thepolydispersity index given by QELS was rather high (>0.2). In fact, amultimodal analysis showed the presence of few aggregates (400-700 nm)contributing to the increase in the polydispersity. These aggregatescould be easily removed by filtration.

TABLE 10 Mean size (determined by quasi-elastic light scatteringmeasurements, in triplicate) of the blank and GF-loaded micellesprepared from the different copolymers. Mean size in nm (PI^(a))Copolymer 0 mg/g^(b) 10 mg/g^(b) 30 mg/g^(b) 40 mg/g^(b) MPEG2-PLA3 63.9± 1.2 (0.50) 70.0 ± 0.5 (0.53) 79.2 ± 0.1 (0.50) 19.3 ± 0.1 (0.10)MPEG2-PmHLA3 77.4 ± 0.6 (0.60) 50.5 ± 0.1 (0.47) 30.4 ± 0.7 (0.30) 43.5± 1.1 (0.31) MPEG-PdiHLA3 29.1 ± 0.2 (0.26) 52.7 ± 0.4 (0.51) 32.5 ± 0.6(0.33) 39.4 ± 0.4 (0.37) ^(a)Polydipsersity (μ₂/Γ²) provided byquasi-elastic light scattering. ^(a)introduced amount of GF in milligramper gram of copolymer for micelle preparation

In order to further characterize the properties of the micelles formedfrom these novel PHLA-based amphiphilic copolymers compared to thoseprepared from the standard MPEG-PLA, Nile Red probe incorporationexperiments were performed. The maximum absorption wavelength of thisdye is strongly influenced by its hydrophobic environment, as alreadyreported by Davis et al. (1966). Nile red solution in each micellesolution (2.1 mg/mL), and the mixtures were slowly agitated for 24hours. The micelle solutions turned quickly reddish as a result of thediffusion of the Nile red into the core of the micelles, whereas controlsolutions with pure PEG and pure water remained uncoloured. UV-visiblespectra were recorded for MPEG2-PLA3, MPEG2-PmHLA3 and MPEG2-PdiHLA3.The maximum wavelength absorption is shifting from 545 nm to 540 nm and535 nm, respectively, indicating clearly that the polarity of themicelle is decreasing as a result of the increasing density of hexylgroups on the PLA chain. Moreover, the Nile Red absorbance was higherwith an increasing number of hexyl groups on the polyester, showing thathigher amounts of the Nile Red hydrophobic molecule were incorporated inthe micelle core. The quasi absence of absorbance observed for thesample made of pure PEG confirmed that the Nile Red has no affinity forthe hydrophilic PEG block, and is only to observe in combination withthe hydrophobic P(H)LA block, which is concentrated in the core of themicelle.

Critical micellar concentrations (CMC) were determined using Nile Red asa fluorescent probe. Based on the fact that this molecule is quasiinsoluble in water and solubilizes itself only into the hydrophobicregion of micelles, an intense fluorescence can be observed as soon asmicelles are formed, as shown in FIG. 10B. Here the maximum fluorescenceintensity is presented as a function of the polymer concentration. TheCMC could be precisely determined at the inflection point of the plot ofthe maximum emission wavelength as a function of the polymerconcentration (FIG. 19A.), a method developed by Coutinho et al. (2002).As expected, the CMC determined for MPEG2-PmHLA3 and MPEG2-PdiHLA3 wasslightly lower with 8.5 and 8 mg/mL, respectively in comparison to theanalog MPEG2-PLA3 with 10 mg/L, due to the increased hydrophobicity ofthe P(m/di)HLA3 segment. These low CMC values allow to envision the useof these novel micelles as drug carriers in very diluted conditions.

The reinforced hydrophobic character of the micelle core of thehexyl-substituted PLA in comparison to standard PLA can especially be ofinterest for optimized hydrophobic drug loadings. For this purpose, theincorporation of griseofulvin (GF) in these novel micelles wasinvestigated. The procedure to prepare the GF-loaded micelles was thesame as for the blank ones, except that GF was dissolved with thecopolymer in acetone prior to addition in the water phase. After removalof acetone, the non-entrapped GF was insoluble in water and easilyeliminated by centrifugation. Whatever the amounts of GF used, the meansize was always comprised in the range of 30-80 nm. The amount ofencapsulated GF in the different micelles was assessed by HPLC afterdestroying the micelles by solubilization of the copolymer inacetonitrile. As shown in FIG. 11, the levels of loaded GF were higherin the micelles with the more hydrophobic core, whatever the introducedamount of GF. A significant difference was observed between PLA-basedmicelles and PmHLA and PdiHLA-based ones. For the two latters, thelevels of encapsulated GF were relatively similar.

This Example provides the synthesis and characterization of novelamphiphilic MPEG-PHLA di-block copolymers by ROP of themonohexyl-substituted lactide using a methoxy-terminated PEG asinitiator and in the presence of Sn(Oct)₂ as a catalyst. Predictablemolecular weights and narrow distributions were achieved. The physicalproperties of the copolymers were determined by DSC, showing thepresence of amorphous and crystalline domains arising from bothhomopolymers. Micelles (˜40-90 nm) were successfully prepared from thesenew copolymers. UV-visible experiments with Nile Red showed a reinforcedhydrophobicity of the micelle core when increasing the density of hexylgroups on the polyester chain, with a shift observed in the maximumabsorption wavelength and higher amounts of Nile Red incorporated. TheCMC was very lower (8-8.5 mg/L) for the micelles based on the mono anddi hexyl-substituted polylactide than that obtained for those based onthe standard PLA (10 mg/mL), and allow to envision the use of thesemicelles as drug carriers in extremely diluted conditions. Thisreinforced hydrophobicity of the inner micelle core led to highhydrophobic drug loadings, as it was shown for the griseofulvin modeldrug. Finally, it is to point out that the copolymer composition andmolecular weight can be easily tuned thanks to the flexibility of theROP, depending on the final properties required for the material.

Example 5 Synthesis and Characterization of Novel Acrylated Star-ShapedPoly(Hexyl Substituted Lactides) for Obtention of Semi-SolidBiodegradable Networks

Biodegradable PLA-based networks have recently received high attentiondue to their possible application as materials in key issues of tissueengineering and drug delivery (Barakat et al., 1996; Kelly et al.,2003). As for now, most of the biodegradable networks are based onacrylated star-shaped PLAs with further crosslinking (Robson et al.,1993; Helminen et al., 2001). The inventors describe here thepossibility of tailoring the properties of such networks, based on theinventors' approach of the synthesis and ring-opening polymerization ofnew alkyl-substituted lactides, as already reported (Trimaille et al.,2004). Particularly, the inventors present here the synthesis andcharacterization of acrylated star-shaped poly(monohexyl-substitutedlactides) leading to semi-solid low glass transition temperaturenetworks.

The monohexyl-substituted lactide (mHLA), whose synthesis is described(Trimaille et al., 2005), was polymerized in the presence of1,1,1-tris(hydroxymethyl)ethane (TE) and pentaerythritol (PE) asmultifunctional alcohol initiators to achieve 3 arms and 4 armsstar-shaped architectures, respectively. The FDA-approved Sn(Oct)₂ wasused as a catalyst at a molar ratio catalyst-to-initiator of 0.25, andROP were performed in convenient bulk conditions at 100° C. The resultsare presented in the Table 11. A first series of mHLA polymerizationswas performed targeting a DP of 30 with TE (entry 2) and PE (entry 3).Conversions were nearly complete (95%) after 3 h reaction for bothinitiators, and predictable molecular weights and narrowpolydispersities were obtained (Mw/Mn<1.20). As a reference, standardD,L-lactide was polymerized in the same conditions (except toluene wasused as a solvent) with TE and the conversion was total (entry 1). Theslightly higher polydispersity obtained (Mw/Mn=1.26) compared to that ofmHLA (1.16) can be attributed to the beginning of sidetransesterification reactions at complete conversion (100%, Table 11).

TABLE 11 Characteristics of the star shaped poly(monohexyl-substitutedlactides) Temp Time Conv. DP M_(n) entry Monomer Initiator C/I (° C.)(h) (%) Targeted Meas.^(b) (g/mol) M_(w)/M_(n) 1 D,L-lac TE 0.25 100^(a) 3 ~100 30 29 5090 1.26 2 mHLA TE 0.25 100 3 95 30 31 5810 1.163 mHLA PE 0.25 100 3 95 30 n.d.^(c) 6500 1.19 4 mHLA TE 0.25 100 5 85 6038 7700 1.24 5 mHLA TE 0.25 100 5 60 120 39 6950 1.31 6 mHLA PE 0.25 1005 85 60 n.d.^(c)  6880^(a) 1.49 7 mHLA PE 0.25 100 5 72 120 n.d.^(c) 7770^(a) 1.35 ^(a)using toluene as solvent ^(b)by ¹H NMR ^(c)notdetermined due to overlapping

¹H NMR was used to confirm the structure the 3-arms and 4-armsstar-shaped PmHLA (CDCl₃, 300 MHz) of DP 30, as shown below.

The ¹H NMR spectra of the PmHLA of targeted DP 30 initiated with TE andPE are presented in FIGS. 23A-B, respectively. All the peaks wereidentified, particularly the CHOH end groups and CH₂ singlets from theTE and PE initiators showing that the latters were effective asinitiating the polymerization and demonstrating the star-shapedarchitecture of the polymer. Nevertheless, for PE-initiatedpolymerizations, the overlapping of these peaks did not allow thecalculation of the DP (from CH₂ and methine protons integrals). ForTE-initiated polymerizations, the calculation was possible and theexperimental DP obtained was really close to the expected one (Table11). The CH₃ singlet (at about 1 ppm) from this initiator was alsoclearly identified.

The control of the polymerization of mHLA with TE and PE was thenfurther investigated by targeting higher DPs of 60 and 120 (Table 11,entries 4-7), with a polymerization time of 5 hours. A loss in thepolymerization control was observed, as evidenced by the lower MW and DPthan that expected and higher polydispersities. Investigating in furtherdetails the GPC chromatograms of the polymers initiated by TE and PE,nice narrow and symmetric peaks were observed for a targeted DP of 30,whereas peaks with a slight shoulder were observed for higher targetedDPs, explaining a higher polydispersity and a lower MW than expected.However, this phenomenon was less marked for the polymerizationsinitiated with TE than for those initiated with PE, with a “shoulder”effect observed only for the highest targeted DP 120 (for PE, it wasobserved for both DPs 60 and 120). This can be attributed to the factthat TE alcohol was good soluble in the melting monomer during thepolymerization whereas PE was insoluble in either melts or solvents astoluene and THF. Initiation with the latter alcohol was thus lessefficient, and as a result all the chains can not be initiated andpropagate at the same time, as required for a “living” polymerizationprocess.

With a view to obtain biodegradable networks from PmHLA, the alcoholendgroups of the polymer were then acrylated through the use of acryloylchloride, as described in Scheme 6. Star-shaped PLA was also tested as areference. The acrylation yield was calculated by ¹H NMR afterpurification (by precipitation in MeOH) comparing the peak integrals ofvinyl protons with the one of the CH₂ originating from the initiator. ¹HNMR was obtained for the purified star-shaped PHmLA before and after theacrylation in the presence of a large excess of acryloyl chloride (˜75eq. per alcohol endgroup of the polymer). Acrylation yield was very good(>95%), and the values of molecular weights and polydispersity weresimilar to those obtained before acrylation (Table 12).

TABLE 12 Characteristics of the acrylated star-shaped (3 arms) PmHLA andPLA polymers. Acrylation yield (%) M_(n) (g/mol) M_(w)/M_(n) Acrylst-PLA 98 4810 1.17 Acryl. st-PmHLA 95 6290 1.09

Crosslinking of the obtained polymer was then performed using theconventional AIBN initiator (10 mol % per double bonds of the polymer).After 1 h reaction in THF at 70° C., the network was formed, as seenfrom the solid mixture insoluble in the THF and swollen by this solvent.The glass transition temperature of the obtained network (−20.5° C.) wassignificantly different of those of the st-PmHLA before or afteracrylation (−13.7 and −14.2° C.), demonstrating the novel properties ofthe obtained network compared to the star-shaped polymer. This workshows the possibility to easily obtain biodegradable semi-solidnetworks.

Monohexyl-substituted lactide was synthesized as previously described(Trimaille et al., 2005). D,L-lactide from Purac Biochem (TheNetherlands) was delivered under vacuum and directly transferred into aglove-box for storage. Tin(II) 2-ethylhexanoate (Sn(Oct)₂),pentaerythritol (PE) and 1,1,1-tris(hydroxymethyl)ethane (TE) werepurchased from Aldrich (Buchs, Switzerland) and acryloyl chloride andα,α′-azoisobutyronitrile (AIBN) were from Fluka (Buchs, Switzerland). PEand TE were carefully dried over vacuum before use. Solvents were driedby standard methods and distilled prior to use.

Star-Shaped P(mH)LA Synthesis.

Polymerizations were typically run with 1 g of monomer in bulk(monohexyl-substituted lactide) or in toluene (D,L-lactide) in thepresence of Sn(Oct)₂ as catalyst and PE or TE as multifunctionalinitiator (molar ratio Sn(Oct)₂/initiator=0.25). A reaction flaskcontaining a stirbar was fitted with a septum, flamed under vacuum, andplaced into a glove-box, where the monomer was filled in. In a typicalprocedure (for a targeted degree of polymerization [DP] of 30), 1.0 gmonohexyl-substituted lactide (9.34 mmol) with 27.7 mg TE was heated formelting, and 120 μL Sn(Oct)₂ stock solution (0.20 g/mL in dry THF) wereadded under argon atmosphere, and the mixture was heated to 100° C. Atthe desired reaction time, ˜100 mg of mixture was taken out and thereaction was quenched by adding non dry THF, followed by precipitationin cold MeOH and drying at 40° C. under vacuum.

Acrylated Star-Shaped P(mH)LA.

The rest of the reaction mixture was quenched by adding a large excessof acryloyl chloride with respect to the alcohol end groups of thepolymer (3 mL) in dry THF. The solution was stirred at room temperaturefor 16 h, and 1 mL water was added to hydrolyze the remaining acryloylchloride. The polymers were precipitated in methanol.

Crosslinking of the Acrylated Star-Shaped P(mH)LA.

The acrylated polymer was dissolved in THF with AIBN (10 mol % perdouble bonds). The solution was degassed with argon for 30 minutes andthen heated to 70° C. The network was formed after 1 h-1 h30(insolubility in THF).

Polymer Characterization.

Polymerization conversions and DP were determined by ¹H NMR analysis,and molecular weights and polydispersities by Gel PermeationChromatography (GPC). The ¹H NMR spectra were recorded in deuteratedchloroform with a Bruker spectrometer (300 MHz). GPC was carried out ona Waters chromatographer, mounted with Styragel HR 1-4 columns (Waters)and connected to a Waters 410 differential refractometer. THF was thecontinuous phase and polystyrenes of known molecular weights: 500, 2630,5970, 9100, 37900, 96400 g/mol (Tosoh Corporation) were used ascalibration standards.

Thermal analysis of the polymers was performed with a differentialscanning calorimeter (SSC/5200, Seiko Instruments). Heating wasperformed at a flow rate of 10° C./min under nitrogen and thetemperature was calibrated with an indium standard.

All of the compositions and/or methods disclosed and claimed herein canbe made and executed without undue experimentation in light of thepresent disclosure. While the compositions and methods of this inventionhave been described in terms of preferred embodiments, it will beapparent to those of skill in the art that variations may be applied tothe compositions and/or methods and in the steps or in the sequence ofsteps of the method described herein without departing from the concept,spirit and scope of the invention. More specifically, it will beapparent that certain agents which are both chemically andphysiologically related may be substituted for the agents describedherein while the same or similar results would be achieved. All suchsimilar substitutes and modifications apparent to those skilled in theart are deemed to be within the spirit, scope and concept of theinvention as defined by the appended claims.

REFERENCES

The following references, to the extent that they provide exemplaryprocedural or other details supplementary to those set forth herein, arespecifically incorporated herein by reference.

-   U.S. Pat. No. 5,792,451-   U.S. Pat. No. 5,399,363-   U.S. Pat. No. 5,466,468-   U.S. Pat. No. 5,466,468-   U.S. Pat. No. 5,543,158-   U.S. Pat. No. 5,580,579-   U.S. Pat. No. 5,629,001-   U.S. Pat. No. 5,641,515-   U.S. Pat. No. 5,641,515-   U.S. Pat. No. 5,725,871-   U.S. Pat. No. 5,756,353-   U.S. Pat. No. 5,780,045-   U.S. Pat. No. 5,804,212-   U.S. Pat. No. 6,126,919-   U.S. Pat. No. 6,469,133-   U.S. Pat. No. 6,613,308-   U.S. Pat. No. 6,753,514-   Amsden et al., Biomacromolecules, 5:637, 2004.-   Barakat et al., J. Polym. Sci. Part A, 37:2401-2411, 1996.-   Bolte et al., Acta Crystall. Sec. C, C50:1717-1721, 1994.-   Chrisholm and Delbridge, J. Chem., 27:1167-1176, 2003.-   Davis et al., Biomaterials, 24(14):2485-2495, 2003.-   Deane and Hammond, J. Dairy Sci., 43:1421-1429, 1960.-   Dechy-Cabaret et al., Chem., Rev., 104:6147-6176, 2004.-   Degee et al., J. Polym. Sci. A, Polym. Chem., 37:2413-2420, 1999.-   Drumright et al., Advanced Materials, 12:1941-1846, 2000.-   Food Drug Admin. Food Additives, Ref. Regist., 40(121)C, 1975.-   Fox and Loshaek, J. Appl. Phys., 26:1080, 1955.-   Grizzi et al., Biomaterials, 16:305-311, 1995.-   Gurny et al., Macromolecules, 32:301-307, 1999.-   Hakkarainen et al., Polym. Deg. Stab., 52:283-291, 1996.-   Hall and Hill, J. Soc. Cosmet. Chem., 37:397-407, 1986.-   Hatefi and Amsden, J. Control Rel., 80:9, 2002.-   Helminen et al., Polymer, 42(8):3345-3353, 2001.-   Hwang et al., Crit. Rev. Ther. Drug Carrier Syst., 15(3):243-284,    1998.-   Hyon et al., Biomaterials., 18:1503-1508, 1997.-   Jamshidi et al., Polymer, 29:2229-2234, 1988.-   Jiang et al., Adv. Drug Deliv. Rev., 57:391-410, 2005.-   Kazunori et al., Advanced Drug Deliv. Revs., 47(1):113-131, 2001.-   Kowalski et al., Macromol. Rapid Commun., 19:567-572, 1998.-   Kricheldorf et al., Macromolecules, 33:702-709, 2000.-   Kricheldorf et al., Polymer, 36:1253-1259, 1995.-   Lin et al., Pharm. Res., 20:668-673, 2003.-   Liu and Ma, Ann. Biomed. Eng., 32:477-486, 2004.-   Lou et al., Macromol. Rapid. Commun., 24:161-172, 2003.-   Mathiowitz et al., Nature, 386(6623):410-414, 1997.-   McGuinness et al., J. Polym. Sci. A Polym. Chem., 41:3798-3803,    2003.-   Merkli et al., J. Control Rel., 29:105-112, 1994.-   Möller et al., J. Polym. Sci. A Polym. Chem., 38:2067-2074, 2000.-   Möller et al., J. Polym. Sci. A Polym. Chem., 39:3529-3538, 2001.-   Mu and Feng, J. Control. Release, 86:33-48, 2003.-   Mullen et al., J. Polym. Sci. A Polym. Chem., 41:1978-1991, 2003.-   Myers et al., J. Polym. Sci. A Polym. Chem., 40:844-851, 2002.-   Nederberg et al., Chem. Int. Ed, 40:2712-2715, 2001.-   Ouchi et al., J. Polym. Sci. A Polym. Chem., 40:1218-1255, 2002.-   Penning et al., Polymer, 34:942-951, 1993.-   Porter and Johnson, Chem. Rev., 66:1, 1966.-   Remington's Pharmaceutical Sciences, 15^(th) ed., pages 1035-1038    and 1570-1580, Mack Publishing Company, Easton, Pa., 1980.-   Remington's Pharmaceutical Sciences, 18th Ed. Mack Printing Company,    1289-1329, 1990.-   Riley et al., Langmuir, 17(11):3168-3174, 2001.-   Robson et al., Polymer, 34(20):4365-4372, 1993.-   Ryner et al., Macromolecules, 34:7281-7287, 2001.-   Schollkopf et al., Chem., 91:329-330, 1979.-   Schwach et al., J. Polym. Sci. A Polym. Chem., 35:3431-3440, 1997.-   Schwach-Abdellaoui et al., Proc. Intern Symp. Control Rel. Bioact.    Mater, 28:3065-306, 2001.-   Schwach-Abdellaoui et al., Biomaterials, 22:1659-1666, 2001.-   Shiosaki and Rapoport, J. Org. Chem., 50:1229-1239, 1985.-   Shirahama et al., J. Polym. Sci. A Polym. Chem., 40:302-316, 2002.-   Simmons and Baker, Biomacromolecules, 2:658-663, 2001.-   Stock and Mayer, Long Ter Eff. Med. Implants, 11:249-260, 2001.-   Storey and Sherman, Macromolecules, 35:1504-1512, 2002.-   Storey et al., J. Polym. Sci. A Polym. Chem., 41:1296-1305, 2003.-   Takenaga et al., J. Control Release, 52(1-2):81-87, 1998.-   Trimaille et al., J. Polym. Sci. A Polym. Chem., 42:4379-4391, 2004.-   Traimaille et al., Chimia, 59:348-352, 2005.-   Tsuji et al., Macromolecules, 24:2719-2724, 1991.-   Uhrich et al., Chem. Rev., 99:3181-3198, 1999.-   Vert et al., In: Macromolecular biomaterials, Hastings et al.    (Eds.), CRS Press, Fl, 119, 1984.-   Vink et al., Polym. Deg. Stab., 80:403-419, 2003.-   Yasugi et al., J. Controlled Release, 62(1-2):89-100, 1999.-   Ye et al., React. Funct. Polym., 32:161-168, 1997.-   Yin and Baker, Macromolecules, 32:7711-7718, 1999.-   Yoo and Park, J. Controller Release, 70(1-2)63-70, 2001.

1.-73. (canceled)
 74. A pharmaceutical composition comprising apolylactide and a drug, wherein the polylactide has the structure:

wherein: R¹, R², R³, and R⁴ are each independently chosen from the groupconsisting of unsubstituted C₁₋₂₀ alkyl and hydrogen; n is 2 to 100; Xis hydrogen; at least one of R¹, R², R³, and R⁴ is C₄₋₂₀ unsubstitutedalkyl; Y is —OH; wherein the pharmaceutical composition is formulatedfor injection; and wherein the polylactide has a T_(g) value of lessthan 15° C.
 75. The composition of claim 74, wherein the pharmaceuticalcomposition is formulated for parenteral, intravenous, intradermal,transdermal, intrathecal, intraarterial, intraperitoneal, intramuscular,infusion, or subcutaneous administration.
 76. The composition of claim75, wherein the pharmaceutical composition is formulated for parenteraladministration.
 77. The composition of claim 75, wherein thepharmaceutical composition is formulated for intramuscularadministration.
 78. The composition of claim 75, wherein thepharmaceutical composition is formulated for subcutaneousadministration.
 79. The composition of claim 74, wherein n is 2 to 75.80. The composition of claim 79, wherein n is 2 to
 50. 81. Thecomposition of claim 80, wherein n is 2 to
 25. 82. The composition ofclaim 74, wherein R¹ and R³ are hydrogen; wherein R² is —(CH₂)_(m)—CH₃,and wherein m is from 3 to
 12. 83. The composition of claim 82, whereinm=4-6.
 84. The composition of claim 82, wherein m=5.
 85. The compositionof claim 79, wherein R¹ and R³ are hydrogen; wherein R⁴ is—(CH₂)_(m)—CH₃, and wherein m is from 3 to
 12. 86. The composition ofclaim 85, wherein m=4-6.
 87. The composition of claim 86, wherein m=5.88. The composition of claim 82, wherein R¹ and R³ are hydrogen; whereinR² and R⁴ are —(CH₂)_(m)—CH₃, and wherein m is from 3 to
 12. 89. Thecomposition of claim 74, wherein R¹ and R³ are hydrogen; and wherein R²and R⁴ are —(CH₂)_(m)—CH₃ or —CH₃, wherein m=3-12.
 90. The compositionof claim 89, wherein m=4-6.
 91. The composition of claim 90, whereinm=5.